1 ============================ 2 LINUX KERNEL MEMORY BARRIERS 3 ============================ 4 5By: David Howells <dhowells@redhat.com> 6 Paul E. McKenney <paulmck@linux.vnet.ibm.com> 7 8Contents: 9 10 (*) Abstract memory access model. 11 12 - Device operations. 13 - Guarantees. 14 15 (*) What are memory barriers? 16 17 - Varieties of memory barrier. 18 - What may not be assumed about memory barriers? 19 - Data dependency barriers. 20 - Control dependencies. 21 - SMP barrier pairing. 22 - Examples of memory barrier sequences. 23 - Read memory barriers vs load speculation. 24 - Transitivity 25 26 (*) Explicit kernel barriers. 27 28 - Compiler barrier. 29 - CPU memory barriers. 30 - MMIO write barrier. 31 32 (*) Implicit kernel memory barriers. 33 34 - Locking functions. 35 - Interrupt disabling functions. 36 - Sleep and wake-up functions. 37 - Miscellaneous functions. 38 39 (*) Inter-CPU locking barrier effects. 40 41 - Locks vs memory accesses. 42 - Locks vs I/O accesses. 43 44 (*) Where are memory barriers needed? 45 46 - Interprocessor interaction. 47 - Atomic operations. 48 - Accessing devices. 49 - Interrupts. 50 51 (*) Kernel I/O barrier effects. 52 53 (*) Assumed minimum execution ordering model. 54 55 (*) The effects of the cpu cache. 56 57 - Cache coherency. 58 - Cache coherency vs DMA. 59 - Cache coherency vs MMIO. 60 61 (*) The things CPUs get up to. 62 63 - And then there's the Alpha. 64 65 (*) Example uses. 66 67 - Circular buffers. 68 69 (*) References. 70 71 72============================ 73ABSTRACT MEMORY ACCESS MODEL 74============================ 75 76Consider the following abstract model of the system: 77 78 : : 79 : : 80 : : 81 +-------+ : +--------+ : +-------+ 82 | | : | | : | | 83 | | : | | : | | 84 | CPU 1 |<----->| Memory |<----->| CPU 2 | 85 | | : | | : | | 86 | | : | | : | | 87 +-------+ : +--------+ : +-------+ 88 ^ : ^ : ^ 89 | : | : | 90 | : | : | 91 | : v : | 92 | : +--------+ : | 93 | : | | : | 94 | : | | : | 95 +---------->| Device |<----------+ 96 : | | : 97 : | | : 98 : +--------+ : 99 : : 100 101Each CPU executes a program that generates memory access operations. In the 102abstract CPU, memory operation ordering is very relaxed, and a CPU may actually 103perform the memory operations in any order it likes, provided program causality 104appears to be maintained. Similarly, the compiler may also arrange the 105instructions it emits in any order it likes, provided it doesn't affect the 106apparent operation of the program. 107 108So in the above diagram, the effects of the memory operations performed by a 109CPU are perceived by the rest of the system as the operations cross the 110interface between the CPU and rest of the system (the dotted lines). 111 112 113For example, consider the following sequence of events: 114 115 CPU 1 CPU 2 116 =============== =============== 117 { A == 1; B == 2 } 118 A = 3; x = A; 119 B = 4; y = B; 120 121The set of accesses as seen by the memory system in the middle can be arranged 122in 24 different combinations: 123 124 STORE A=3, STORE B=4, x=LOAD A->3, y=LOAD B->4 125 STORE A=3, STORE B=4, y=LOAD B->4, x=LOAD A->3 126 STORE A=3, x=LOAD A->3, STORE B=4, y=LOAD B->4 127 STORE A=3, x=LOAD A->3, y=LOAD B->2, STORE B=4 128 STORE A=3, y=LOAD B->2, STORE B=4, x=LOAD A->3 129 STORE A=3, y=LOAD B->2, x=LOAD A->3, STORE B=4 130 STORE B=4, STORE A=3, x=LOAD A->3, y=LOAD B->4 131 STORE B=4, ... 132 ... 133 134and can thus result in four different combinations of values: 135 136 x == 1, y == 2 137 x == 1, y == 4 138 x == 3, y == 2 139 x == 3, y == 4 140 141 142Furthermore, the stores committed by a CPU to the memory system may not be 143perceived by the loads made by another CPU in the same order as the stores were 144committed. 145 146 147As a further example, consider this sequence of events: 148 149 CPU 1 CPU 2 150 =============== =============== 151 { A == 1, B == 2, C = 3, P == &A, Q == &C } 152 B = 4; Q = P; 153 P = &B D = *Q; 154 155There is an obvious data dependency here, as the value loaded into D depends on 156the address retrieved from P by CPU 2. At the end of the sequence, any of the 157following results are possible: 158 159 (Q == &A) and (D == 1) 160 (Q == &B) and (D == 2) 161 (Q == &B) and (D == 4) 162 163Note that CPU 2 will never try and load C into D because the CPU will load P 164into Q before issuing the load of *Q. 165 166 167DEVICE OPERATIONS 168----------------- 169 170Some devices present their control interfaces as collections of memory 171locations, but the order in which the control registers are accessed is very 172important. For instance, imagine an ethernet card with a set of internal 173registers that are accessed through an address port register (A) and a data 174port register (D). To read internal register 5, the following code might then 175be used: 176 177 *A = 5; 178 x = *D; 179 180but this might show up as either of the following two sequences: 181 182 STORE *A = 5, x = LOAD *D 183 x = LOAD *D, STORE *A = 5 184 185the second of which will almost certainly result in a malfunction, since it set 186the address _after_ attempting to read the register. 187 188 189GUARANTEES 190---------- 191 192There are some minimal guarantees that may be expected of a CPU: 193 194 (*) On any given CPU, dependent memory accesses will be issued in order, with 195 respect to itself. This means that for: 196 197 Q = P; D = *Q; 198 199 the CPU will issue the following memory operations: 200 201 Q = LOAD P, D = LOAD *Q 202 203 and always in that order. 204 205 (*) Overlapping loads and stores within a particular CPU will appear to be 206 ordered within that CPU. This means that for: 207 208 a = *X; *X = b; 209 210 the CPU will only issue the following sequence of memory operations: 211 212 a = LOAD *X, STORE *X = b 213 214 And for: 215 216 *X = c; d = *X; 217 218 the CPU will only issue: 219 220 STORE *X = c, d = LOAD *X 221 222 (Loads and stores overlap if they are targeted at overlapping pieces of 223 memory). 224 225And there are a number of things that _must_ or _must_not_ be assumed: 226 227 (*) It _must_not_ be assumed that independent loads and stores will be issued 228 in the order given. This means that for: 229 230 X = *A; Y = *B; *D = Z; 231 232 we may get any of the following sequences: 233 234 X = LOAD *A, Y = LOAD *B, STORE *D = Z 235 X = LOAD *A, STORE *D = Z, Y = LOAD *B 236 Y = LOAD *B, X = LOAD *A, STORE *D = Z 237 Y = LOAD *B, STORE *D = Z, X = LOAD *A 238 STORE *D = Z, X = LOAD *A, Y = LOAD *B 239 STORE *D = Z, Y = LOAD *B, X = LOAD *A 240 241 (*) It _must_ be assumed that overlapping memory accesses may be merged or 242 discarded. This means that for: 243 244 X = *A; Y = *(A + 4); 245 246 we may get any one of the following sequences: 247 248 X = LOAD *A; Y = LOAD *(A + 4); 249 Y = LOAD *(A + 4); X = LOAD *A; 250 {X, Y} = LOAD {*A, *(A + 4) }; 251 252 And for: 253 254 *A = X; *(A + 4) = Y; 255 256 we may get any of: 257 258 STORE *A = X; STORE *(A + 4) = Y; 259 STORE *(A + 4) = Y; STORE *A = X; 260 STORE {*A, *(A + 4) } = {X, Y}; 261 262 263========================= 264WHAT ARE MEMORY BARRIERS? 265========================= 266 267As can be seen above, independent memory operations are effectively performed 268in random order, but this can be a problem for CPU-CPU interaction and for I/O. 269What is required is some way of intervening to instruct the compiler and the 270CPU to restrict the order. 271 272Memory barriers are such interventions. They impose a perceived partial 273ordering over the memory operations on either side of the barrier. 274 275Such enforcement is important because the CPUs and other devices in a system 276can use a variety of tricks to improve performance, including reordering, 277deferral and combination of memory operations; speculative loads; speculative 278branch prediction and various types of caching. Memory barriers are used to 279override or suppress these tricks, allowing the code to sanely control the 280interaction of multiple CPUs and/or devices. 281 282 283VARIETIES OF MEMORY BARRIER 284--------------------------- 285 286Memory barriers come in four basic varieties: 287 288 (1) Write (or store) memory barriers. 289 290 A write memory barrier gives a guarantee that all the STORE operations 291 specified before the barrier will appear to happen before all the STORE 292 operations specified after the barrier with respect to the other 293 components of the system. 294 295 A write barrier is a partial ordering on stores only; it is not required 296 to have any effect on loads. 297 298 A CPU can be viewed as committing a sequence of store operations to the 299 memory system as time progresses. All stores before a write barrier will 300 occur in the sequence _before_ all the stores after the write barrier. 301 302 [!] Note that write barriers should normally be paired with read or data 303 dependency barriers; see the "SMP barrier pairing" subsection. 304 305 306 (2) Data dependency barriers. 307 308 A data dependency barrier is a weaker form of read barrier. In the case 309 where two loads are performed such that the second depends on the result 310 of the first (eg: the first load retrieves the address to which the second 311 load will be directed), a data dependency barrier would be required to 312 make sure that the target of the second load is updated before the address 313 obtained by the first load is accessed. 314 315 A data dependency barrier is a partial ordering on interdependent loads 316 only; it is not required to have any effect on stores, independent loads 317 or overlapping loads. 318 319 As mentioned in (1), the other CPUs in the system can be viewed as 320 committing sequences of stores to the memory system that the CPU being 321 considered can then perceive. A data dependency barrier issued by the CPU 322 under consideration guarantees that for any load preceding it, if that 323 load touches one of a sequence of stores from another CPU, then by the 324 time the barrier completes, the effects of all the stores prior to that 325 touched by the load will be perceptible to any loads issued after the data 326 dependency barrier. 327 328 See the "Examples of memory barrier sequences" subsection for diagrams 329 showing the ordering constraints. 330 331 [!] Note that the first load really has to have a _data_ dependency and 332 not a control dependency. If the address for the second load is dependent 333 on the first load, but the dependency is through a conditional rather than 334 actually loading the address itself, then it's a _control_ dependency and 335 a full read barrier or better is required. See the "Control dependencies" 336 subsection for more information. 337 338 [!] Note that data dependency barriers should normally be paired with 339 write barriers; see the "SMP barrier pairing" subsection. 340 341 342 (3) Read (or load) memory barriers. 343 344 A read barrier is a data dependency barrier plus a guarantee that all the 345 LOAD operations specified before the barrier will appear to happen before 346 all the LOAD operations specified after the barrier with respect to the 347 other components of the system. 348 349 A read barrier is a partial ordering on loads only; it is not required to 350 have any effect on stores. 351 352 Read memory barriers imply data dependency barriers, and so can substitute 353 for them. 354 355 [!] Note that read barriers should normally be paired with write barriers; 356 see the "SMP barrier pairing" subsection. 357 358 359 (4) General memory barriers. 360 361 A general memory barrier gives a guarantee that all the LOAD and STORE 362 operations specified before the barrier will appear to happen before all 363 the LOAD and STORE operations specified after the barrier with respect to 364 the other components of the system. 365 366 A general memory barrier is a partial ordering over both loads and stores. 367 368 General memory barriers imply both read and write memory barriers, and so 369 can substitute for either. 370 371 372And a couple of implicit varieties: 373 374 (5) LOCK operations. 375 376 This acts as a one-way permeable barrier. It guarantees that all memory 377 operations after the LOCK operation will appear to happen after the LOCK 378 operation with respect to the other components of the system. 379 380 Memory operations that occur before a LOCK operation may appear to happen 381 after it completes. 382 383 A LOCK operation should almost always be paired with an UNLOCK operation. 384 385 386 (6) UNLOCK operations. 387 388 This also acts as a one-way permeable barrier. It guarantees that all 389 memory operations before the UNLOCK operation will appear to happen before 390 the UNLOCK operation with respect to the other components of the system. 391 392 Memory operations that occur after an UNLOCK operation may appear to 393 happen before it completes. 394 395 LOCK and UNLOCK operations are guaranteed to appear with respect to each 396 other strictly in the order specified. 397 398 The use of LOCK and UNLOCK operations generally precludes the need for 399 other sorts of memory barrier (but note the exceptions mentioned in the 400 subsection "MMIO write barrier"). 401 402 403Memory barriers are only required where there's a possibility of interaction 404between two CPUs or between a CPU and a device. If it can be guaranteed that 405there won't be any such interaction in any particular piece of code, then 406memory barriers are unnecessary in that piece of code. 407 408 409Note that these are the _minimum_ guarantees. Different architectures may give 410more substantial guarantees, but they may _not_ be relied upon outside of arch 411specific code. 412 413 414WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS? 415---------------------------------------------- 416 417There are certain things that the Linux kernel memory barriers do not guarantee: 418 419 (*) There is no guarantee that any of the memory accesses specified before a 420 memory barrier will be _complete_ by the completion of a memory barrier 421 instruction; the barrier can be considered to draw a line in that CPU's 422 access queue that accesses of the appropriate type may not cross. 423 424 (*) There is no guarantee that issuing a memory barrier on one CPU will have 425 any direct effect on another CPU or any other hardware in the system. The 426 indirect effect will be the order in which the second CPU sees the effects 427 of the first CPU's accesses occur, but see the next point: 428 429 (*) There is no guarantee that a CPU will see the correct order of effects 430 from a second CPU's accesses, even _if_ the second CPU uses a memory 431 barrier, unless the first CPU _also_ uses a matching memory barrier (see 432 the subsection on "SMP Barrier Pairing"). 433 434 (*) There is no guarantee that some intervening piece of off-the-CPU 435 hardware[*] will not reorder the memory accesses. CPU cache coherency 436 mechanisms should propagate the indirect effects of a memory barrier 437 between CPUs, but might not do so in order. 438 439 [*] For information on bus mastering DMA and coherency please read: 440 441 Documentation/PCI/pci.txt 442 Documentation/DMA-API-HOWTO.txt 443 Documentation/DMA-API.txt 444 445 446DATA DEPENDENCY BARRIERS 447------------------------ 448 449The usage requirements of data dependency barriers are a little subtle, and 450it's not always obvious that they're needed. To illustrate, consider the 451following sequence of events: 452 453 CPU 1 CPU 2 454 =============== =============== 455 { A == 1, B == 2, C = 3, P == &A, Q == &C } 456 B = 4; 457 <write barrier> 458 P = &B 459 Q = P; 460 D = *Q; 461 462There's a clear data dependency here, and it would seem that by the end of the 463sequence, Q must be either &A or &B, and that: 464 465 (Q == &A) implies (D == 1) 466 (Q == &B) implies (D == 4) 467 468But! CPU 2's perception of P may be updated _before_ its perception of B, thus 469leading to the following situation: 470 471 (Q == &B) and (D == 2) ???? 472 473Whilst this may seem like a failure of coherency or causality maintenance, it 474isn't, and this behaviour can be observed on certain real CPUs (such as the DEC 475Alpha). 476 477To deal with this, a data dependency barrier or better must be inserted 478between the address load and the data load: 479 480 CPU 1 CPU 2 481 =============== =============== 482 { A == 1, B == 2, C = 3, P == &A, Q == &C } 483 B = 4; 484 <write barrier> 485 P = &B 486 Q = P; 487 <data dependency barrier> 488 D = *Q; 489 490This enforces the occurrence of one of the two implications, and prevents the 491third possibility from arising. 492 493[!] Note that this extremely counterintuitive situation arises most easily on 494machines with split caches, so that, for example, one cache bank processes 495even-numbered cache lines and the other bank processes odd-numbered cache 496lines. The pointer P might be stored in an odd-numbered cache line, and the 497variable B might be stored in an even-numbered cache line. Then, if the 498even-numbered bank of the reading CPU's cache is extremely busy while the 499odd-numbered bank is idle, one can see the new value of the pointer P (&B), 500but the old value of the variable B (2). 501 502 503Another example of where data dependency barriers might by required is where a 504number is read from memory and then used to calculate the index for an array 505access: 506 507 CPU 1 CPU 2 508 =============== =============== 509 { M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 } 510 M[1] = 4; 511 <write barrier> 512 P = 1 513 Q = P; 514 <data dependency barrier> 515 D = M[Q]; 516 517 518The data dependency barrier is very important to the RCU system, for example. 519See rcu_dereference() in include/linux/rcupdate.h. This permits the current 520target of an RCU'd pointer to be replaced with a new modified target, without 521the replacement target appearing to be incompletely initialised. 522 523See also the subsection on "Cache Coherency" for a more thorough example. 524 525 526CONTROL DEPENDENCIES 527-------------------- 528 529A control dependency requires a full read memory barrier, not simply a data 530dependency barrier to make it work correctly. Consider the following bit of 531code: 532 533 q = &a; 534 if (p) 535 q = &b; 536 <data dependency barrier> 537 x = *q; 538 539This will not have the desired effect because there is no actual data 540dependency, but rather a control dependency that the CPU may short-circuit by 541attempting to predict the outcome in advance. In such a case what's actually 542required is: 543 544 q = &a; 545 if (p) 546 q = &b; 547 <read barrier> 548 x = *q; 549 550 551SMP BARRIER PAIRING 552------------------- 553 554When dealing with CPU-CPU interactions, certain types of memory barrier should 555always be paired. A lack of appropriate pairing is almost certainly an error. 556 557A write barrier should always be paired with a data dependency barrier or read 558barrier, though a general barrier would also be viable. Similarly a read 559barrier or a data dependency barrier should always be paired with at least an 560write barrier, though, again, a general barrier is viable: 561 562 CPU 1 CPU 2 563 =============== =============== 564 a = 1; 565 <write barrier> 566 b = 2; x = b; 567 <read barrier> 568 y = a; 569 570Or: 571 572 CPU 1 CPU 2 573 =============== =============================== 574 a = 1; 575 <write barrier> 576 b = &a; x = b; 577 <data dependency barrier> 578 y = *x; 579 580Basically, the read barrier always has to be there, even though it can be of 581the "weaker" type. 582 583[!] Note that the stores before the write barrier would normally be expected to 584match the loads after the read barrier or the data dependency barrier, and vice 585versa: 586 587 CPU 1 CPU 2 588 =============== =============== 589 a = 1; }---- --->{ v = c 590 b = 2; } \ / { w = d 591 <write barrier> \ <read barrier> 592 c = 3; } / \ { x = a; 593 d = 4; }---- --->{ y = b; 594 595 596EXAMPLES OF MEMORY BARRIER SEQUENCES 597------------------------------------ 598 599Firstly, write barriers act as partial orderings on store operations. 600Consider the following sequence of events: 601 602 CPU 1 603 ======================= 604 STORE A = 1 605 STORE B = 2 606 STORE C = 3 607 <write barrier> 608 STORE D = 4 609 STORE E = 5 610 611This sequence of events is committed to the memory coherence system in an order 612that the rest of the system might perceive as the unordered set of { STORE A, 613STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E 614}: 615 616 +-------+ : : 617 | | +------+ 618 | |------>| C=3 | } /\ 619 | | : +------+ }----- \ -----> Events perceptible to 620 | | : | A=1 | } \/ the rest of the system 621 | | : +------+ } 622 | CPU 1 | : | B=2 | } 623 | | +------+ } 624 | | wwwwwwwwwwwwwwww } <--- At this point the write barrier 625 | | +------+ } requires all stores prior to the 626 | | : | E=5 | } barrier to be committed before 627 | | : +------+ } further stores may take place 628 | |------>| D=4 | } 629 | | +------+ 630 +-------+ : : 631 | 632 | Sequence in which stores are committed to the 633 | memory system by CPU 1 634 V 635 636 637Secondly, data dependency barriers act as partial orderings on data-dependent 638loads. Consider the following sequence of events: 639 640 CPU 1 CPU 2 641 ======================= ======================= 642 { B = 7; X = 9; Y = 8; C = &Y } 643 STORE A = 1 644 STORE B = 2 645 <write barrier> 646 STORE C = &B LOAD X 647 STORE D = 4 LOAD C (gets &B) 648 LOAD *C (reads B) 649 650Without intervention, CPU 2 may perceive the events on CPU 1 in some 651effectively random order, despite the write barrier issued by CPU 1: 652 653 +-------+ : : : : 654 | | +------+ +-------+ | Sequence of update 655 | |------>| B=2 |----- --->| Y->8 | | of perception on 656 | | : +------+ \ +-------+ | CPU 2 657 | CPU 1 | : | A=1 | \ --->| C->&Y | V 658 | | +------+ | +-------+ 659 | | wwwwwwwwwwwwwwww | : : 660 | | +------+ | : : 661 | | : | C=&B |--- | : : +-------+ 662 | | : +------+ \ | +-------+ | | 663 | |------>| D=4 | ----------->| C->&B |------>| | 664 | | +------+ | +-------+ | | 665 +-------+ : : | : : | | 666 | : : | | 667 | : : | CPU 2 | 668 | +-------+ | | 669 Apparently incorrect ---> | | B->7 |------>| | 670 perception of B (!) | +-------+ | | 671 | : : | | 672 | +-------+ | | 673 The load of X holds ---> \ | X->9 |------>| | 674 up the maintenance \ +-------+ | | 675 of coherence of B ----->| B->2 | +-------+ 676 +-------+ 677 : : 678 679 680In the above example, CPU 2 perceives that B is 7, despite the load of *C 681(which would be B) coming after the LOAD of C. 682 683If, however, a data dependency barrier were to be placed between the load of C 684and the load of *C (ie: B) on CPU 2: 685 686 CPU 1 CPU 2 687 ======================= ======================= 688 { B = 7; X = 9; Y = 8; C = &Y } 689 STORE A = 1 690 STORE B = 2 691 <write barrier> 692 STORE C = &B LOAD X 693 STORE D = 4 LOAD C (gets &B) 694 <data dependency barrier> 695 LOAD *C (reads B) 696 697then the following will occur: 698 699 +-------+ : : : : 700 | | +------+ +-------+ 701 | |------>| B=2 |----- --->| Y->8 | 702 | | : +------+ \ +-------+ 703 | CPU 1 | : | A=1 | \ --->| C->&Y | 704 | | +------+ | +-------+ 705 | | wwwwwwwwwwwwwwww | : : 706 | | +------+ | : : 707 | | : | C=&B |--- | : : +-------+ 708 | | : +------+ \ | +-------+ | | 709 | |------>| D=4 | ----------->| C->&B |------>| | 710 | | +------+ | +-------+ | | 711 +-------+ : : | : : | | 712 | : : | | 713 | : : | CPU 2 | 714 | +-------+ | | 715 | | X->9 |------>| | 716 | +-------+ | | 717 Makes sure all effects ---> \ ddddddddddddddddd | | 718 prior to the store of C \ +-------+ | | 719 are perceptible to ----->| B->2 |------>| | 720 subsequent loads +-------+ | | 721 : : +-------+ 722 723 724And thirdly, a read barrier acts as a partial order on loads. Consider the 725following sequence of events: 726 727 CPU 1 CPU 2 728 ======================= ======================= 729 { A = 0, B = 9 } 730 STORE A=1 731 <write barrier> 732 STORE B=2 733 LOAD B 734 LOAD A 735 736Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in 737some effectively random order, despite the write barrier issued by CPU 1: 738 739 +-------+ : : : : 740 | | +------+ +-------+ 741 | |------>| A=1 |------ --->| A->0 | 742 | | +------+ \ +-------+ 743 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | 744 | | +------+ | +-------+ 745 | |------>| B=2 |--- | : : 746 | | +------+ \ | : : +-------+ 747 +-------+ : : \ | +-------+ | | 748 ---------->| B->2 |------>| | 749 | +-------+ | CPU 2 | 750 | | A->0 |------>| | 751 | +-------+ | | 752 | : : +-------+ 753 \ : : 754 \ +-------+ 755 ---->| A->1 | 756 +-------+ 757 : : 758 759 760If, however, a read barrier were to be placed between the load of B and the 761load of A on CPU 2: 762 763 CPU 1 CPU 2 764 ======================= ======================= 765 { A = 0, B = 9 } 766 STORE A=1 767 <write barrier> 768 STORE B=2 769 LOAD B 770 <read barrier> 771 LOAD A 772 773then the partial ordering imposed by CPU 1 will be perceived correctly by CPU 7742: 775 776 +-------+ : : : : 777 | | +------+ +-------+ 778 | |------>| A=1 |------ --->| A->0 | 779 | | +------+ \ +-------+ 780 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | 781 | | +------+ | +-------+ 782 | |------>| B=2 |--- | : : 783 | | +------+ \ | : : +-------+ 784 +-------+ : : \ | +-------+ | | 785 ---------->| B->2 |------>| | 786 | +-------+ | CPU 2 | 787 | : : | | 788 | : : | | 789 At this point the read ----> \ rrrrrrrrrrrrrrrrr | | 790 barrier causes all effects \ +-------+ | | 791 prior to the storage of B ---->| A->1 |------>| | 792 to be perceptible to CPU 2 +-------+ | | 793 : : +-------+ 794 795 796To illustrate this more completely, consider what could happen if the code 797contained a load of A either side of the read barrier: 798 799 CPU 1 CPU 2 800 ======================= ======================= 801 { A = 0, B = 9 } 802 STORE A=1 803 <write barrier> 804 STORE B=2 805 LOAD B 806 LOAD A [first load of A] 807 <read barrier> 808 LOAD A [second load of A] 809 810Even though the two loads of A both occur after the load of B, they may both 811come up with different values: 812 813 +-------+ : : : : 814 | | +------+ +-------+ 815 | |------>| A=1 |------ --->| A->0 | 816 | | +------+ \ +-------+ 817 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | 818 | | +------+ | +-------+ 819 | |------>| B=2 |--- | : : 820 | | +------+ \ | : : +-------+ 821 +-------+ : : \ | +-------+ | | 822 ---------->| B->2 |------>| | 823 | +-------+ | CPU 2 | 824 | : : | | 825 | : : | | 826 | +-------+ | | 827 | | A->0 |------>| 1st | 828 | +-------+ | | 829 At this point the read ----> \ rrrrrrrrrrrrrrrrr | | 830 barrier causes all effects \ +-------+ | | 831 prior to the storage of B ---->| A->1 |------>| 2nd | 832 to be perceptible to CPU 2 +-------+ | | 833 : : +-------+ 834 835 836But it may be that the update to A from CPU 1 becomes perceptible to CPU 2 837before the read barrier completes anyway: 838 839 +-------+ : : : : 840 | | +------+ +-------+ 841 | |------>| A=1 |------ --->| A->0 | 842 | | +------+ \ +-------+ 843 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | 844 | | +------+ | +-------+ 845 | |------>| B=2 |--- | : : 846 | | +------+ \ | : : +-------+ 847 +-------+ : : \ | +-------+ | | 848 ---------->| B->2 |------>| | 849 | +-------+ | CPU 2 | 850 | : : | | 851 \ : : | | 852 \ +-------+ | | 853 ---->| A->1 |------>| 1st | 854 +-------+ | | 855 rrrrrrrrrrrrrrrrr | | 856 +-------+ | | 857 | A->1 |------>| 2nd | 858 +-------+ | | 859 : : +-------+ 860 861 862The guarantee is that the second load will always come up with A == 1 if the 863load of B came up with B == 2. No such guarantee exists for the first load of 864A; that may come up with either A == 0 or A == 1. 865 866 867READ MEMORY BARRIERS VS LOAD SPECULATION 868---------------------------------------- 869 870Many CPUs speculate with loads: that is they see that they will need to load an 871item from memory, and they find a time where they're not using the bus for any 872other loads, and so do the load in advance - even though they haven't actually 873got to that point in the instruction execution flow yet. This permits the 874actual load instruction to potentially complete immediately because the CPU 875already has the value to hand. 876 877It may turn out that the CPU didn't actually need the value - perhaps because a 878branch circumvented the load - in which case it can discard the value or just 879cache it for later use. 880 881Consider: 882 883 CPU 1 CPU 2 884 ======================= ======================= 885 LOAD B 886 DIVIDE } Divide instructions generally 887 DIVIDE } take a long time to perform 888 LOAD A 889 890Which might appear as this: 891 892 : : +-------+ 893 +-------+ | | 894 --->| B->2 |------>| | 895 +-------+ | CPU 2 | 896 : :DIVIDE | | 897 +-------+ | | 898 The CPU being busy doing a ---> --->| A->0 |~~~~ | | 899 division speculates on the +-------+ ~ | | 900 LOAD of A : : ~ | | 901 : :DIVIDE | | 902 : : ~ | | 903 Once the divisions are complete --> : : ~-->| | 904 the CPU can then perform the : : | | 905 LOAD with immediate effect : : +-------+ 906 907 908Placing a read barrier or a data dependency barrier just before the second 909load: 910 911 CPU 1 CPU 2 912 ======================= ======================= 913 LOAD B 914 DIVIDE 915 DIVIDE 916 <read barrier> 917 LOAD A 918 919will force any value speculatively obtained to be reconsidered to an extent 920dependent on the type of barrier used. If there was no change made to the 921speculated memory location, then the speculated value will just be used: 922 923 : : +-------+ 924 +-------+ | | 925 --->| B->2 |------>| | 926 +-------+ | CPU 2 | 927 : :DIVIDE | | 928 +-------+ | | 929 The CPU being busy doing a ---> --->| A->0 |~~~~ | | 930 division speculates on the +-------+ ~ | | 931 LOAD of A : : ~ | | 932 : :DIVIDE | | 933 : : ~ | | 934 : : ~ | | 935 rrrrrrrrrrrrrrrr~ | | 936 : : ~ | | 937 : : ~-->| | 938 : : | | 939 : : +-------+ 940 941 942but if there was an update or an invalidation from another CPU pending, then 943the speculation will be cancelled and the value reloaded: 944 945 : : +-------+ 946 +-------+ | | 947 --->| B->2 |------>| | 948 +-------+ | CPU 2 | 949 : :DIVIDE | | 950 +-------+ | | 951 The CPU being busy doing a ---> --->| A->0 |~~~~ | | 952 division speculates on the +-------+ ~ | | 953 LOAD of A : : ~ | | 954 : :DIVIDE | | 955 : : ~ | | 956 : : ~ | | 957 rrrrrrrrrrrrrrrrr | | 958 +-------+ | | 959 The speculation is discarded ---> --->| A->1 |------>| | 960 and an updated value is +-------+ | | 961 retrieved : : +-------+ 962 963 964TRANSITIVITY 965------------ 966 967Transitivity is a deeply intuitive notion about ordering that is not 968always provided by real computer systems. The following example 969demonstrates transitivity (also called "cumulativity"): 970 971 CPU 1 CPU 2 CPU 3 972 ======================= ======================= ======================= 973 { X = 0, Y = 0 } 974 STORE X=1 LOAD X STORE Y=1 975 <general barrier> <general barrier> 976 LOAD Y LOAD X 977 978Suppose that CPU 2's load from X returns 1 and its load from Y returns 0. 979This indicates that CPU 2's load from X in some sense follows CPU 1's 980store to X and that CPU 2's load from Y in some sense preceded CPU 3's 981store to Y. The question is then "Can CPU 3's load from X return 0?" 982 983Because CPU 2's load from X in some sense came after CPU 1's store, it 984is natural to expect that CPU 3's load from X must therefore return 1. 985This expectation is an example of transitivity: if a load executing on 986CPU A follows a load from the same variable executing on CPU B, then 987CPU A's load must either return the same value that CPU B's load did, 988or must return some later value. 989 990In the Linux kernel, use of general memory barriers guarantees 991transitivity. Therefore, in the above example, if CPU 2's load from X 992returns 1 and its load from Y returns 0, then CPU 3's load from X must 993also return 1. 994 995However, transitivity is -not- guaranteed for read or write barriers. 996For example, suppose that CPU 2's general barrier in the above example 997is changed to a read barrier as shown below: 998 999 CPU 1 CPU 2 CPU 3 1000 ======================= ======================= ======================= 1001 { X = 0, Y = 0 } 1002 STORE X=1 LOAD X STORE Y=1 1003 <read barrier> <general barrier> 1004 LOAD Y LOAD X 1005 1006This substitution destroys transitivity: in this example, it is perfectly 1007legal for CPU 2's load from X to return 1, its load from Y to return 0, 1008and CPU 3's load from X to return 0. 1009 1010The key point is that although CPU 2's read barrier orders its pair 1011of loads, it does not guarantee to order CPU 1's store. Therefore, if 1012this example runs on a system where CPUs 1 and 2 share a store buffer 1013or a level of cache, CPU 2 might have early access to CPU 1's writes. 1014General barriers are therefore required to ensure that all CPUs agree 1015on the combined order of CPU 1's and CPU 2's accesses. 1016 1017To reiterate, if your code requires transitivity, use general barriers 1018throughout. 1019 1020 1021======================== 1022EXPLICIT KERNEL BARRIERS 1023======================== 1024 1025The Linux kernel has a variety of different barriers that act at different 1026levels: 1027 1028 (*) Compiler barrier. 1029 1030 (*) CPU memory barriers. 1031 1032 (*) MMIO write barrier. 1033 1034 1035COMPILER BARRIER 1036---------------- 1037 1038The Linux kernel has an explicit compiler barrier function that prevents the 1039compiler from moving the memory accesses either side of it to the other side: 1040 1041 barrier(); 1042 1043This is a general barrier - lesser varieties of compiler barrier do not exist. 1044 1045The compiler barrier has no direct effect on the CPU, which may then reorder 1046things however it wishes. 1047 1048 1049CPU MEMORY BARRIERS 1050------------------- 1051 1052The Linux kernel has eight basic CPU memory barriers: 1053 1054 TYPE MANDATORY SMP CONDITIONAL 1055 =============== ======================= =========================== 1056 GENERAL mb() smp_mb() 1057 WRITE wmb() smp_wmb() 1058 READ rmb() smp_rmb() 1059 DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends() 1060 1061 1062All memory barriers except the data dependency barriers imply a compiler 1063barrier. Data dependencies do not impose any additional compiler ordering. 1064 1065Aside: In the case of data dependencies, the compiler would be expected to 1066issue the loads in the correct order (eg. `a[b]` would have to load the value 1067of b before loading a[b]), however there is no guarantee in the C specification 1068that the compiler may not speculate the value of b (eg. is equal to 1) and load 1069a before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the 1070problem of a compiler reloading b after having loaded a[b], thus having a newer 1071copy of b than a[b]. A consensus has not yet been reached about these problems, 1072however the ACCESS_ONCE macro is a good place to start looking. 1073 1074SMP memory barriers are reduced to compiler barriers on uniprocessor compiled 1075systems because it is assumed that a CPU will appear to be self-consistent, 1076and will order overlapping accesses correctly with respect to itself. 1077 1078[!] Note that SMP memory barriers _must_ be used to control the ordering of 1079references to shared memory on SMP systems, though the use of locking instead 1080is sufficient. 1081 1082Mandatory barriers should not be used to control SMP effects, since mandatory 1083barriers unnecessarily impose overhead on UP systems. They may, however, be 1084used to control MMIO effects on accesses through relaxed memory I/O windows. 1085These are required even on non-SMP systems as they affect the order in which 1086memory operations appear to a device by prohibiting both the compiler and the 1087CPU from reordering them. 1088 1089 1090There are some more advanced barrier functions: 1091 1092 (*) set_mb(var, value) 1093 1094 This assigns the value to the variable and then inserts a full memory 1095 barrier after it, depending on the function. It isn't guaranteed to 1096 insert anything more than a compiler barrier in a UP compilation. 1097 1098 1099 (*) smp_mb__before_atomic_dec(); 1100 (*) smp_mb__after_atomic_dec(); 1101 (*) smp_mb__before_atomic_inc(); 1102 (*) smp_mb__after_atomic_inc(); 1103 1104 These are for use with atomic add, subtract, increment and decrement 1105 functions that don't return a value, especially when used for reference 1106 counting. These functions do not imply memory barriers. 1107 1108 As an example, consider a piece of code that marks an object as being dead 1109 and then decrements the object's reference count: 1110 1111 obj->dead = 1; 1112 smp_mb__before_atomic_dec(); 1113 atomic_dec(&obj->ref_count); 1114 1115 This makes sure that the death mark on the object is perceived to be set 1116 *before* the reference counter is decremented. 1117 1118 See Documentation/atomic_ops.txt for more information. See the "Atomic 1119 operations" subsection for information on where to use these. 1120 1121 1122 (*) smp_mb__before_clear_bit(void); 1123 (*) smp_mb__after_clear_bit(void); 1124 1125 These are for use similar to the atomic inc/dec barriers. These are 1126 typically used for bitwise unlocking operations, so care must be taken as 1127 there are no implicit memory barriers here either. 1128 1129 Consider implementing an unlock operation of some nature by clearing a 1130 locking bit. The clear_bit() would then need to be barriered like this: 1131 1132 smp_mb__before_clear_bit(); 1133 clear_bit( ... ); 1134 1135 This prevents memory operations before the clear leaking to after it. See 1136 the subsection on "Locking Functions" with reference to UNLOCK operation 1137 implications. 1138 1139 See Documentation/atomic_ops.txt for more information. See the "Atomic 1140 operations" subsection for information on where to use these. 1141 1142 1143MMIO WRITE BARRIER 1144------------------ 1145 1146The Linux kernel also has a special barrier for use with memory-mapped I/O 1147writes: 1148 1149 mmiowb(); 1150 1151This is a variation on the mandatory write barrier that causes writes to weakly 1152ordered I/O regions to be partially ordered. Its effects may go beyond the 1153CPU->Hardware interface and actually affect the hardware at some level. 1154 1155See the subsection "Locks vs I/O accesses" for more information. 1156 1157 1158=============================== 1159IMPLICIT KERNEL MEMORY BARRIERS 1160=============================== 1161 1162Some of the other functions in the linux kernel imply memory barriers, amongst 1163which are locking and scheduling functions. 1164 1165This specification is a _minimum_ guarantee; any particular architecture may 1166provide more substantial guarantees, but these may not be relied upon outside 1167of arch specific code. 1168 1169 1170LOCKING FUNCTIONS 1171----------------- 1172 1173The Linux kernel has a number of locking constructs: 1174 1175 (*) spin locks 1176 (*) R/W spin locks 1177 (*) mutexes 1178 (*) semaphores 1179 (*) R/W semaphores 1180 (*) RCU 1181 1182In all cases there are variants on "LOCK" operations and "UNLOCK" operations 1183for each construct. These operations all imply certain barriers: 1184 1185 (1) LOCK operation implication: 1186 1187 Memory operations issued after the LOCK will be completed after the LOCK 1188 operation has completed. 1189 1190 Memory operations issued before the LOCK may be completed after the LOCK 1191 operation has completed. 1192 1193 (2) UNLOCK operation implication: 1194 1195 Memory operations issued before the UNLOCK will be completed before the 1196 UNLOCK operation has completed. 1197 1198 Memory operations issued after the UNLOCK may be completed before the 1199 UNLOCK operation has completed. 1200 1201 (3) LOCK vs LOCK implication: 1202 1203 All LOCK operations issued before another LOCK operation will be completed 1204 before that LOCK operation. 1205 1206 (4) LOCK vs UNLOCK implication: 1207 1208 All LOCK operations issued before an UNLOCK operation will be completed 1209 before the UNLOCK operation. 1210 1211 All UNLOCK operations issued before a LOCK operation will be completed 1212 before the LOCK operation. 1213 1214 (5) Failed conditional LOCK implication: 1215 1216 Certain variants of the LOCK operation may fail, either due to being 1217 unable to get the lock immediately, or due to receiving an unblocked 1218 signal whilst asleep waiting for the lock to become available. Failed 1219 locks do not imply any sort of barrier. 1220 1221Therefore, from (1), (2) and (4) an UNLOCK followed by an unconditional LOCK is 1222equivalent to a full barrier, but a LOCK followed by an UNLOCK is not. 1223 1224[!] Note: one of the consequences of LOCKs and UNLOCKs being only one-way 1225 barriers is that the effects of instructions outside of a critical section 1226 may seep into the inside of the critical section. 1227 1228A LOCK followed by an UNLOCK may not be assumed to be full memory barrier 1229because it is possible for an access preceding the LOCK to happen after the 1230LOCK, and an access following the UNLOCK to happen before the UNLOCK, and the 1231two accesses can themselves then cross: 1232 1233 *A = a; 1234 LOCK 1235 UNLOCK 1236 *B = b; 1237 1238may occur as: 1239 1240 LOCK, STORE *B, STORE *A, UNLOCK 1241 1242Locks and semaphores may not provide any guarantee of ordering on UP compiled 1243systems, and so cannot be counted on in such a situation to actually achieve 1244anything at all - especially with respect to I/O accesses - unless combined 1245with interrupt disabling operations. 1246 1247See also the section on "Inter-CPU locking barrier effects". 1248 1249 1250As an example, consider the following: 1251 1252 *A = a; 1253 *B = b; 1254 LOCK 1255 *C = c; 1256 *D = d; 1257 UNLOCK 1258 *E = e; 1259 *F = f; 1260 1261The following sequence of events is acceptable: 1262 1263 LOCK, {*F,*A}, *E, {*C,*D}, *B, UNLOCK 1264 1265 [+] Note that {*F,*A} indicates a combined access. 1266 1267But none of the following are: 1268 1269 {*F,*A}, *B, LOCK, *C, *D, UNLOCK, *E 1270 *A, *B, *C, LOCK, *D, UNLOCK, *E, *F 1271 *A, *B, LOCK, *C, UNLOCK, *D, *E, *F 1272 *B, LOCK, *C, *D, UNLOCK, {*F,*A}, *E 1273 1274 1275 1276INTERRUPT DISABLING FUNCTIONS 1277----------------------------- 1278 1279Functions that disable interrupts (LOCK equivalent) and enable interrupts 1280(UNLOCK equivalent) will act as compiler barriers only. So if memory or I/O 1281barriers are required in such a situation, they must be provided from some 1282other means. 1283 1284 1285SLEEP AND WAKE-UP FUNCTIONS 1286--------------------------- 1287 1288Sleeping and waking on an event flagged in global data can be viewed as an 1289interaction between two pieces of data: the task state of the task waiting for 1290the event and the global data used to indicate the event. To make sure that 1291these appear to happen in the right order, the primitives to begin the process 1292of going to sleep, and the primitives to initiate a wake up imply certain 1293barriers. 1294 1295Firstly, the sleeper normally follows something like this sequence of events: 1296 1297 for (;;) { 1298 set_current_state(TASK_UNINTERRUPTIBLE); 1299 if (event_indicated) 1300 break; 1301 schedule(); 1302 } 1303 1304A general memory barrier is interpolated automatically by set_current_state() 1305after it has altered the task state: 1306 1307 CPU 1 1308 =============================== 1309 set_current_state(); 1310 set_mb(); 1311 STORE current->state 1312 <general barrier> 1313 LOAD event_indicated 1314 1315set_current_state() may be wrapped by: 1316 1317 prepare_to_wait(); 1318 prepare_to_wait_exclusive(); 1319 1320which therefore also imply a general memory barrier after setting the state. 1321The whole sequence above is available in various canned forms, all of which 1322interpolate the memory barrier in the right place: 1323 1324 wait_event(); 1325 wait_event_interruptible(); 1326 wait_event_interruptible_exclusive(); 1327 wait_event_interruptible_timeout(); 1328 wait_event_killable(); 1329 wait_event_timeout(); 1330 wait_on_bit(); 1331 wait_on_bit_lock(); 1332 1333 1334Secondly, code that performs a wake up normally follows something like this: 1335 1336 event_indicated = 1; 1337 wake_up(&event_wait_queue); 1338 1339or: 1340 1341 event_indicated = 1; 1342 wake_up_process(event_daemon); 1343 1344A write memory barrier is implied by wake_up() and co. if and only if they wake 1345something up. The barrier occurs before the task state is cleared, and so sits 1346between the STORE to indicate the event and the STORE to set TASK_RUNNING: 1347 1348 CPU 1 CPU 2 1349 =============================== =============================== 1350 set_current_state(); STORE event_indicated 1351 set_mb(); wake_up(); 1352 STORE current->state <write barrier> 1353 <general barrier> STORE current->state 1354 LOAD event_indicated 1355 1356The available waker functions include: 1357 1358 complete(); 1359 wake_up(); 1360 wake_up_all(); 1361 wake_up_bit(); 1362 wake_up_interruptible(); 1363 wake_up_interruptible_all(); 1364 wake_up_interruptible_nr(); 1365 wake_up_interruptible_poll(); 1366 wake_up_interruptible_sync(); 1367 wake_up_interruptible_sync_poll(); 1368 wake_up_locked(); 1369 wake_up_locked_poll(); 1370 wake_up_nr(); 1371 wake_up_poll(); 1372 wake_up_process(); 1373 1374 1375[!] Note that the memory barriers implied by the sleeper and the waker do _not_ 1376order multiple stores before the wake-up with respect to loads of those stored 1377values after the sleeper has called set_current_state(). For instance, if the 1378sleeper does: 1379 1380 set_current_state(TASK_INTERRUPTIBLE); 1381 if (event_indicated) 1382 break; 1383 __set_current_state(TASK_RUNNING); 1384 do_something(my_data); 1385 1386and the waker does: 1387 1388 my_data = value; 1389 event_indicated = 1; 1390 wake_up(&event_wait_queue); 1391 1392there's no guarantee that the change to event_indicated will be perceived by 1393the sleeper as coming after the change to my_data. In such a circumstance, the 1394code on both sides must interpolate its own memory barriers between the 1395separate data accesses. Thus the above sleeper ought to do: 1396 1397 set_current_state(TASK_INTERRUPTIBLE); 1398 if (event_indicated) { 1399 smp_rmb(); 1400 do_something(my_data); 1401 } 1402 1403and the waker should do: 1404 1405 my_data = value; 1406 smp_wmb(); 1407 event_indicated = 1; 1408 wake_up(&event_wait_queue); 1409 1410 1411MISCELLANEOUS FUNCTIONS 1412----------------------- 1413 1414Other functions that imply barriers: 1415 1416 (*) schedule() and similar imply full memory barriers. 1417 1418 1419================================= 1420INTER-CPU LOCKING BARRIER EFFECTS 1421================================= 1422 1423On SMP systems locking primitives give a more substantial form of barrier: one 1424that does affect memory access ordering on other CPUs, within the context of 1425conflict on any particular lock. 1426 1427 1428LOCKS VS MEMORY ACCESSES 1429------------------------ 1430 1431Consider the following: the system has a pair of spinlocks (M) and (Q), and 1432three CPUs; then should the following sequence of events occur: 1433 1434 CPU 1 CPU 2 1435 =============================== =============================== 1436 *A = a; *E = e; 1437 LOCK M LOCK Q 1438 *B = b; *F = f; 1439 *C = c; *G = g; 1440 UNLOCK M UNLOCK Q 1441 *D = d; *H = h; 1442 1443Then there is no guarantee as to what order CPU 3 will see the accesses to *A 1444through *H occur in, other than the constraints imposed by the separate locks 1445on the separate CPUs. It might, for example, see: 1446 1447 *E, LOCK M, LOCK Q, *G, *C, *F, *A, *B, UNLOCK Q, *D, *H, UNLOCK M 1448 1449But it won't see any of: 1450 1451 *B, *C or *D preceding LOCK M 1452 *A, *B or *C following UNLOCK M 1453 *F, *G or *H preceding LOCK Q 1454 *E, *F or *G following UNLOCK Q 1455 1456 1457However, if the following occurs: 1458 1459 CPU 1 CPU 2 1460 =============================== =============================== 1461 *A = a; 1462 LOCK M [1] 1463 *B = b; 1464 *C = c; 1465 UNLOCK M [1] 1466 *D = d; *E = e; 1467 LOCK M [2] 1468 *F = f; 1469 *G = g; 1470 UNLOCK M [2] 1471 *H = h; 1472 1473CPU 3 might see: 1474 1475 *E, LOCK M [1], *C, *B, *A, UNLOCK M [1], 1476 LOCK M [2], *H, *F, *G, UNLOCK M [2], *D 1477 1478But assuming CPU 1 gets the lock first, CPU 3 won't see any of: 1479 1480 *B, *C, *D, *F, *G or *H preceding LOCK M [1] 1481 *A, *B or *C following UNLOCK M [1] 1482 *F, *G or *H preceding LOCK M [2] 1483 *A, *B, *C, *E, *F or *G following UNLOCK M [2] 1484 1485 1486LOCKS VS I/O ACCESSES 1487--------------------- 1488 1489Under certain circumstances (especially involving NUMA), I/O accesses within 1490two spinlocked sections on two different CPUs may be seen as interleaved by the 1491PCI bridge, because the PCI bridge does not necessarily participate in the 1492cache-coherence protocol, and is therefore incapable of issuing the required 1493read memory barriers. 1494 1495For example: 1496 1497 CPU 1 CPU 2 1498 =============================== =============================== 1499 spin_lock(Q) 1500 writel(0, ADDR) 1501 writel(1, DATA); 1502 spin_unlock(Q); 1503 spin_lock(Q); 1504 writel(4, ADDR); 1505 writel(5, DATA); 1506 spin_unlock(Q); 1507 1508may be seen by the PCI bridge as follows: 1509 1510 STORE *ADDR = 0, STORE *ADDR = 4, STORE *DATA = 1, STORE *DATA = 5 1511 1512which would probably cause the hardware to malfunction. 1513 1514 1515What is necessary here is to intervene with an mmiowb() before dropping the 1516spinlock, for example: 1517 1518 CPU 1 CPU 2 1519 =============================== =============================== 1520 spin_lock(Q) 1521 writel(0, ADDR) 1522 writel(1, DATA); 1523 mmiowb(); 1524 spin_unlock(Q); 1525 spin_lock(Q); 1526 writel(4, ADDR); 1527 writel(5, DATA); 1528 mmiowb(); 1529 spin_unlock(Q); 1530 1531this will ensure that the two stores issued on CPU 1 appear at the PCI bridge 1532before either of the stores issued on CPU 2. 1533 1534 1535Furthermore, following a store by a load from the same device obviates the need 1536for the mmiowb(), because the load forces the store to complete before the load 1537is performed: 1538 1539 CPU 1 CPU 2 1540 =============================== =============================== 1541 spin_lock(Q) 1542 writel(0, ADDR) 1543 a = readl(DATA); 1544 spin_unlock(Q); 1545 spin_lock(Q); 1546 writel(4, ADDR); 1547 b = readl(DATA); 1548 spin_unlock(Q); 1549 1550 1551See Documentation/DocBook/deviceiobook.tmpl for more information. 1552 1553 1554================================= 1555WHERE ARE MEMORY BARRIERS NEEDED? 1556================================= 1557 1558Under normal operation, memory operation reordering is generally not going to 1559be a problem as a single-threaded linear piece of code will still appear to 1560work correctly, even if it's in an SMP kernel. There are, however, four 1561circumstances in which reordering definitely _could_ be a problem: 1562 1563 (*) Interprocessor interaction. 1564 1565 (*) Atomic operations. 1566 1567 (*) Accessing devices. 1568 1569 (*) Interrupts. 1570 1571 1572INTERPROCESSOR INTERACTION 1573-------------------------- 1574 1575When there's a system with more than one processor, more than one CPU in the 1576system may be working on the same data set at the same time. This can cause 1577synchronisation problems, and the usual way of dealing with them is to use 1578locks. Locks, however, are quite expensive, and so it may be preferable to 1579operate without the use of a lock if at all possible. In such a case 1580operations that affect both CPUs may have to be carefully ordered to prevent 1581a malfunction. 1582 1583Consider, for example, the R/W semaphore slow path. Here a waiting process is 1584queued on the semaphore, by virtue of it having a piece of its stack linked to 1585the semaphore's list of waiting processes: 1586 1587 struct rw_semaphore { 1588 ... 1589 spinlock_t lock; 1590 struct list_head waiters; 1591 }; 1592 1593 struct rwsem_waiter { 1594 struct list_head list; 1595 struct task_struct *task; 1596 }; 1597 1598To wake up a particular waiter, the up_read() or up_write() functions have to: 1599 1600 (1) read the next pointer from this waiter's record to know as to where the 1601 next waiter record is; 1602 1603 (2) read the pointer to the waiter's task structure; 1604 1605 (3) clear the task pointer to tell the waiter it has been given the semaphore; 1606 1607 (4) call wake_up_process() on the task; and 1608 1609 (5) release the reference held on the waiter's task struct. 1610 1611In other words, it has to perform this sequence of events: 1612 1613 LOAD waiter->list.next; 1614 LOAD waiter->task; 1615 STORE waiter->task; 1616 CALL wakeup 1617 RELEASE task 1618 1619and if any of these steps occur out of order, then the whole thing may 1620malfunction. 1621 1622Once it has queued itself and dropped the semaphore lock, the waiter does not 1623get the lock again; it instead just waits for its task pointer to be cleared 1624before proceeding. Since the record is on the waiter's stack, this means that 1625if the task pointer is cleared _before_ the next pointer in the list is read, 1626another CPU might start processing the waiter and might clobber the waiter's 1627stack before the up*() function has a chance to read the next pointer. 1628 1629Consider then what might happen to the above sequence of events: 1630 1631 CPU 1 CPU 2 1632 =============================== =============================== 1633 down_xxx() 1634 Queue waiter 1635 Sleep 1636 up_yyy() 1637 LOAD waiter->task; 1638 STORE waiter->task; 1639 Woken up by other event 1640 <preempt> 1641 Resume processing 1642 down_xxx() returns 1643 call foo() 1644 foo() clobbers *waiter 1645 </preempt> 1646 LOAD waiter->list.next; 1647 --- OOPS --- 1648 1649This could be dealt with using the semaphore lock, but then the down_xxx() 1650function has to needlessly get the spinlock again after being woken up. 1651 1652The way to deal with this is to insert a general SMP memory barrier: 1653 1654 LOAD waiter->list.next; 1655 LOAD waiter->task; 1656 smp_mb(); 1657 STORE waiter->task; 1658 CALL wakeup 1659 RELEASE task 1660 1661In this case, the barrier makes a guarantee that all memory accesses before the 1662barrier will appear to happen before all the memory accesses after the barrier 1663with respect to the other CPUs on the system. It does _not_ guarantee that all 1664the memory accesses before the barrier will be complete by the time the barrier 1665instruction itself is complete. 1666 1667On a UP system - where this wouldn't be a problem - the smp_mb() is just a 1668compiler barrier, thus making sure the compiler emits the instructions in the 1669right order without actually intervening in the CPU. Since there's only one 1670CPU, that CPU's dependency ordering logic will take care of everything else. 1671 1672 1673ATOMIC OPERATIONS 1674----------------- 1675 1676Whilst they are technically interprocessor interaction considerations, atomic 1677operations are noted specially as some of them imply full memory barriers and 1678some don't, but they're very heavily relied on as a group throughout the 1679kernel. 1680 1681Any atomic operation that modifies some state in memory and returns information 1682about the state (old or new) implies an SMP-conditional general memory barrier 1683(smp_mb()) on each side of the actual operation (with the exception of 1684explicit lock operations, described later). These include: 1685 1686 xchg(); 1687 cmpxchg(); 1688 atomic_xchg(); 1689 atomic_cmpxchg(); 1690 atomic_inc_return(); 1691 atomic_dec_return(); 1692 atomic_add_return(); 1693 atomic_sub_return(); 1694 atomic_inc_and_test(); 1695 atomic_dec_and_test(); 1696 atomic_sub_and_test(); 1697 atomic_add_negative(); 1698 atomic_add_unless(); /* when succeeds (returns 1) */ 1699 test_and_set_bit(); 1700 test_and_clear_bit(); 1701 test_and_change_bit(); 1702 1703These are used for such things as implementing LOCK-class and UNLOCK-class 1704operations and adjusting reference counters towards object destruction, and as 1705such the implicit memory barrier effects are necessary. 1706 1707 1708The following operations are potential problems as they do _not_ imply memory 1709barriers, but might be used for implementing such things as UNLOCK-class 1710operations: 1711 1712 atomic_set(); 1713 set_bit(); 1714 clear_bit(); 1715 change_bit(); 1716 1717With these the appropriate explicit memory barrier should be used if necessary 1718(smp_mb__before_clear_bit() for instance). 1719 1720 1721The following also do _not_ imply memory barriers, and so may require explicit 1722memory barriers under some circumstances (smp_mb__before_atomic_dec() for 1723instance): 1724 1725 atomic_add(); 1726 atomic_sub(); 1727 atomic_inc(); 1728 atomic_dec(); 1729 1730If they're used for statistics generation, then they probably don't need memory 1731barriers, unless there's a coupling between statistical data. 1732 1733If they're used for reference counting on an object to control its lifetime, 1734they probably don't need memory barriers because either the reference count 1735will be adjusted inside a locked section, or the caller will already hold 1736sufficient references to make the lock, and thus a memory barrier unnecessary. 1737 1738If they're used for constructing a lock of some description, then they probably 1739do need memory barriers as a lock primitive generally has to do things in a 1740specific order. 1741 1742Basically, each usage case has to be carefully considered as to whether memory 1743barriers are needed or not. 1744 1745The following operations are special locking primitives: 1746 1747 test_and_set_bit_lock(); 1748 clear_bit_unlock(); 1749 __clear_bit_unlock(); 1750 1751These implement LOCK-class and UNLOCK-class operations. These should be used in 1752preference to other operations when implementing locking primitives, because 1753their implementations can be optimised on many architectures. 1754 1755[!] Note that special memory barrier primitives are available for these 1756situations because on some CPUs the atomic instructions used imply full memory 1757barriers, and so barrier instructions are superfluous in conjunction with them, 1758and in such cases the special barrier primitives will be no-ops. 1759 1760See Documentation/atomic_ops.txt for more information. 1761 1762 1763ACCESSING DEVICES 1764----------------- 1765 1766Many devices can be memory mapped, and so appear to the CPU as if they're just 1767a set of memory locations. To control such a device, the driver usually has to 1768make the right memory accesses in exactly the right order. 1769 1770However, having a clever CPU or a clever compiler creates a potential problem 1771in that the carefully sequenced accesses in the driver code won't reach the 1772device in the requisite order if the CPU or the compiler thinks it is more 1773efficient to reorder, combine or merge accesses - something that would cause 1774the device to malfunction. 1775 1776Inside of the Linux kernel, I/O should be done through the appropriate accessor 1777routines - such as inb() or writel() - which know how to make such accesses 1778appropriately sequential. Whilst this, for the most part, renders the explicit 1779use of memory barriers unnecessary, there are a couple of situations where they 1780might be needed: 1781 1782 (1) On some systems, I/O stores are not strongly ordered across all CPUs, and 1783 so for _all_ general drivers locks should be used and mmiowb() must be 1784 issued prior to unlocking the critical section. 1785 1786 (2) If the accessor functions are used to refer to an I/O memory window with 1787 relaxed memory access properties, then _mandatory_ memory barriers are 1788 required to enforce ordering. 1789 1790See Documentation/DocBook/deviceiobook.tmpl for more information. 1791 1792 1793INTERRUPTS 1794---------- 1795 1796A driver may be interrupted by its own interrupt service routine, and thus the 1797two parts of the driver may interfere with each other's attempts to control or 1798access the device. 1799 1800This may be alleviated - at least in part - by disabling local interrupts (a 1801form of locking), such that the critical operations are all contained within 1802the interrupt-disabled section in the driver. Whilst the driver's interrupt 1803routine is executing, the driver's core may not run on the same CPU, and its 1804interrupt is not permitted to happen again until the current interrupt has been 1805handled, thus the interrupt handler does not need to lock against that. 1806 1807However, consider a driver that was talking to an ethernet card that sports an 1808address register and a data register. If that driver's core talks to the card 1809under interrupt-disablement and then the driver's interrupt handler is invoked: 1810 1811 LOCAL IRQ DISABLE 1812 writew(ADDR, 3); 1813 writew(DATA, y); 1814 LOCAL IRQ ENABLE 1815 <interrupt> 1816 writew(ADDR, 4); 1817 q = readw(DATA); 1818 </interrupt> 1819 1820The store to the data register might happen after the second store to the 1821address register if ordering rules are sufficiently relaxed: 1822 1823 STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA 1824 1825 1826If ordering rules are relaxed, it must be assumed that accesses done inside an 1827interrupt disabled section may leak outside of it and may interleave with 1828accesses performed in an interrupt - and vice versa - unless implicit or 1829explicit barriers are used. 1830 1831Normally this won't be a problem because the I/O accesses done inside such 1832sections will include synchronous load operations on strictly ordered I/O 1833registers that form implicit I/O barriers. If this isn't sufficient then an 1834mmiowb() may need to be used explicitly. 1835 1836 1837A similar situation may occur between an interrupt routine and two routines 1838running on separate CPUs that communicate with each other. If such a case is 1839likely, then interrupt-disabling locks should be used to guarantee ordering. 1840 1841 1842========================== 1843KERNEL I/O BARRIER EFFECTS 1844========================== 1845 1846When accessing I/O memory, drivers should use the appropriate accessor 1847functions: 1848 1849 (*) inX(), outX(): 1850 1851 These are intended to talk to I/O space rather than memory space, but 1852 that's primarily a CPU-specific concept. The i386 and x86_64 processors do 1853 indeed have special I/O space access cycles and instructions, but many 1854 CPUs don't have such a concept. 1855 1856 The PCI bus, amongst others, defines an I/O space concept which - on such 1857 CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O 1858 space. However, it may also be mapped as a virtual I/O space in the CPU's 1859 memory map, particularly on those CPUs that don't support alternate I/O 1860 spaces. 1861 1862 Accesses to this space may be fully synchronous (as on i386), but 1863 intermediary bridges (such as the PCI host bridge) may not fully honour 1864 that. 1865 1866 They are guaranteed to be fully ordered with respect to each other. 1867 1868 They are not guaranteed to be fully ordered with respect to other types of 1869 memory and I/O operation. 1870 1871 (*) readX(), writeX(): 1872 1873 Whether these are guaranteed to be fully ordered and uncombined with 1874 respect to each other on the issuing CPU depends on the characteristics 1875 defined for the memory window through which they're accessing. On later 1876 i386 architecture machines, for example, this is controlled by way of the 1877 MTRR registers. 1878 1879 Ordinarily, these will be guaranteed to be fully ordered and uncombined, 1880 provided they're not accessing a prefetchable device. 1881 1882 However, intermediary hardware (such as a PCI bridge) may indulge in 1883 deferral if it so wishes; to flush a store, a load from the same location 1884 is preferred[*], but a load from the same device or from configuration 1885 space should suffice for PCI. 1886 1887 [*] NOTE! attempting to load from the same location as was written to may 1888 cause a malfunction - consider the 16550 Rx/Tx serial registers for 1889 example. 1890 1891 Used with prefetchable I/O memory, an mmiowb() barrier may be required to 1892 force stores to be ordered. 1893 1894 Please refer to the PCI specification for more information on interactions 1895 between PCI transactions. 1896 1897 (*) readX_relaxed() 1898 1899 These are similar to readX(), but are not guaranteed to be ordered in any 1900 way. Be aware that there is no I/O read barrier available. 1901 1902 (*) ioreadX(), iowriteX() 1903 1904 These will perform appropriately for the type of access they're actually 1905 doing, be it inX()/outX() or readX()/writeX(). 1906 1907 1908======================================== 1909ASSUMED MINIMUM EXECUTION ORDERING MODEL 1910======================================== 1911 1912It has to be assumed that the conceptual CPU is weakly-ordered but that it will 1913maintain the appearance of program causality with respect to itself. Some CPUs 1914(such as i386 or x86_64) are more constrained than others (such as powerpc or 1915frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside 1916of arch-specific code. 1917 1918This means that it must be considered that the CPU will execute its instruction 1919stream in any order it feels like - or even in parallel - provided that if an 1920instruction in the stream depends on an earlier instruction, then that 1921earlier instruction must be sufficiently complete[*] before the later 1922instruction may proceed; in other words: provided that the appearance of 1923causality is maintained. 1924 1925 [*] Some instructions have more than one effect - such as changing the 1926 condition codes, changing registers or changing memory - and different 1927 instructions may depend on different effects. 1928 1929A CPU may also discard any instruction sequence that winds up having no 1930ultimate effect. For example, if two adjacent instructions both load an 1931immediate value into the same register, the first may be discarded. 1932 1933 1934Similarly, it has to be assumed that compiler might reorder the instruction 1935stream in any way it sees fit, again provided the appearance of causality is 1936maintained. 1937 1938 1939============================ 1940THE EFFECTS OF THE CPU CACHE 1941============================ 1942 1943The way cached memory operations are perceived across the system is affected to 1944a certain extent by the caches that lie between CPUs and memory, and by the 1945memory coherence system that maintains the consistency of state in the system. 1946 1947As far as the way a CPU interacts with another part of the system through the 1948caches goes, the memory system has to include the CPU's caches, and memory 1949barriers for the most part act at the interface between the CPU and its cache 1950(memory barriers logically act on the dotted line in the following diagram): 1951 1952 <--- CPU ---> : <----------- Memory -----------> 1953 : 1954 +--------+ +--------+ : +--------+ +-----------+ 1955 | | | | : | | | | +--------+ 1956 | CPU | | Memory | : | CPU | | | | | 1957 | Core |--->| Access |----->| Cache |<-->| | | | 1958 | | | Queue | : | | | |--->| Memory | 1959 | | | | : | | | | | | 1960 +--------+ +--------+ : +--------+ | | | | 1961 : | Cache | +--------+ 1962 : | Coherency | 1963 : | Mechanism | +--------+ 1964 +--------+ +--------+ : +--------+ | | | | 1965 | | | | : | | | | | | 1966 | CPU | | Memory | : | CPU | | |--->| Device | 1967 | Core |--->| Access |----->| Cache |<-->| | | | 1968 | | | Queue | : | | | | | | 1969 | | | | : | | | | +--------+ 1970 +--------+ +--------+ : +--------+ +-----------+ 1971 : 1972 : 1973 1974Although any particular load or store may not actually appear outside of the 1975CPU that issued it since it may have been satisfied within the CPU's own cache, 1976it will still appear as if the full memory access had taken place as far as the 1977other CPUs are concerned since the cache coherency mechanisms will migrate the 1978cacheline over to the accessing CPU and propagate the effects upon conflict. 1979 1980The CPU core may execute instructions in any order it deems fit, provided the 1981expected program causality appears to be maintained. Some of the instructions 1982generate load and store operations which then go into the queue of memory 1983accesses to be performed. The core may place these in the queue in any order 1984it wishes, and continue execution until it is forced to wait for an instruction 1985to complete. 1986 1987What memory barriers are concerned with is controlling the order in which 1988accesses cross from the CPU side of things to the memory side of things, and 1989the order in which the effects are perceived to happen by the other observers 1990in the system. 1991 1992[!] Memory barriers are _not_ needed within a given CPU, as CPUs always see 1993their own loads and stores as if they had happened in program order. 1994 1995[!] MMIO or other device accesses may bypass the cache system. This depends on 1996the properties of the memory window through which devices are accessed and/or 1997the use of any special device communication instructions the CPU may have. 1998 1999 2000CACHE COHERENCY 2001--------------- 2002 2003Life isn't quite as simple as it may appear above, however: for while the 2004caches are expected to be coherent, there's no guarantee that that coherency 2005will be ordered. This means that whilst changes made on one CPU will 2006eventually become visible on all CPUs, there's no guarantee that they will 2007become apparent in the same order on those other CPUs. 2008 2009 2010Consider dealing with a system that has a pair of CPUs (1 & 2), each of which 2011has a pair of parallel data caches (CPU 1 has A/B, and CPU 2 has C/D): 2012 2013 : 2014 : +--------+ 2015 : +---------+ | | 2016 +--------+ : +--->| Cache A |<------->| | 2017 | | : | +---------+ | | 2018 | CPU 1 |<---+ | | 2019 | | : | +---------+ | | 2020 +--------+ : +--->| Cache B |<------->| | 2021 : +---------+ | | 2022 : | Memory | 2023 : +---------+ | System | 2024 +--------+ : +--->| Cache C |<------->| | 2025 | | : | +---------+ | | 2026 | CPU 2 |<---+ | | 2027 | | : | +---------+ | | 2028 +--------+ : +--->| Cache D |<------->| | 2029 : +---------+ | | 2030 : +--------+ 2031 : 2032 2033Imagine the system has the following properties: 2034 2035 (*) an odd-numbered cache line may be in cache A, cache C or it may still be 2036 resident in memory; 2037 2038 (*) an even-numbered cache line may be in cache B, cache D or it may still be 2039 resident in memory; 2040 2041 (*) whilst the CPU core is interrogating one cache, the other cache may be 2042 making use of the bus to access the rest of the system - perhaps to 2043 displace a dirty cacheline or to do a speculative load; 2044 2045 (*) each cache has a queue of operations that need to be applied to that cache 2046 to maintain coherency with the rest of the system; 2047 2048 (*) the coherency queue is not flushed by normal loads to lines already 2049 present in the cache, even though the contents of the queue may 2050 potentially affect those loads. 2051 2052Imagine, then, that two writes are made on the first CPU, with a write barrier 2053between them to guarantee that they will appear to reach that CPU's caches in 2054the requisite order: 2055 2056 CPU 1 CPU 2 COMMENT 2057 =============== =============== ======================================= 2058 u == 0, v == 1 and p == &u, q == &u 2059 v = 2; 2060 smp_wmb(); Make sure change to v is visible before 2061 change to p 2062 <A:modify v=2> v is now in cache A exclusively 2063 p = &v; 2064 <B:modify p=&v> p is now in cache B exclusively 2065 2066The write memory barrier forces the other CPUs in the system to perceive that 2067the local CPU's caches have apparently been updated in the correct order. But 2068now imagine that the second CPU wants to read those values: 2069 2070 CPU 1 CPU 2 COMMENT 2071 =============== =============== ======================================= 2072 ... 2073 q = p; 2074 x = *q; 2075 2076The above pair of reads may then fail to happen in the expected order, as the 2077cacheline holding p may get updated in one of the second CPU's caches whilst 2078the update to the cacheline holding v is delayed in the other of the second 2079CPU's caches by some other cache event: 2080 2081 CPU 1 CPU 2 COMMENT 2082 =============== =============== ======================================= 2083 u == 0, v == 1 and p == &u, q == &u 2084 v = 2; 2085 smp_wmb(); 2086 <A:modify v=2> <C:busy> 2087 <C:queue v=2> 2088 p = &v; q = p; 2089 <D:request p> 2090 <B:modify p=&v> <D:commit p=&v> 2091 <D:read p> 2092 x = *q; 2093 <C:read *q> Reads from v before v updated in cache 2094 <C:unbusy> 2095 <C:commit v=2> 2096 2097Basically, whilst both cachelines will be updated on CPU 2 eventually, there's 2098no guarantee that, without intervention, the order of update will be the same 2099as that committed on CPU 1. 2100 2101 2102To intervene, we need to interpolate a data dependency barrier or a read 2103barrier between the loads. This will force the cache to commit its coherency 2104queue before processing any further requests: 2105 2106 CPU 1 CPU 2 COMMENT 2107 =============== =============== ======================================= 2108 u == 0, v == 1 and p == &u, q == &u 2109 v = 2; 2110 smp_wmb(); 2111 <A:modify v=2> <C:busy> 2112 <C:queue v=2> 2113 p = &v; q = p; 2114 <D:request p> 2115 <B:modify p=&v> <D:commit p=&v> 2116 <D:read p> 2117 smp_read_barrier_depends() 2118 <C:unbusy> 2119 <C:commit v=2> 2120 x = *q; 2121 <C:read *q> Reads from v after v updated in cache 2122 2123 2124This sort of problem can be encountered on DEC Alpha processors as they have a 2125split cache that improves performance by making better use of the data bus. 2126Whilst most CPUs do imply a data dependency barrier on the read when a memory 2127access depends on a read, not all do, so it may not be relied on. 2128 2129Other CPUs may also have split caches, but must coordinate between the various 2130cachelets for normal memory accesses. The semantics of the Alpha removes the 2131need for coordination in the absence of memory barriers. 2132 2133 2134CACHE COHERENCY VS DMA 2135---------------------- 2136 2137Not all systems maintain cache coherency with respect to devices doing DMA. In 2138such cases, a device attempting DMA may obtain stale data from RAM because 2139dirty cache lines may be resident in the caches of various CPUs, and may not 2140have been written back to RAM yet. To deal with this, the appropriate part of 2141the kernel must flush the overlapping bits of cache on each CPU (and maybe 2142invalidate them as well). 2143 2144In addition, the data DMA'd to RAM by a device may be overwritten by dirty 2145cache lines being written back to RAM from a CPU's cache after the device has 2146installed its own data, or cache lines present in the CPU's cache may simply 2147obscure the fact that RAM has been updated, until at such time as the cacheline 2148is discarded from the CPU's cache and reloaded. To deal with this, the 2149appropriate part of the kernel must invalidate the overlapping bits of the 2150cache on each CPU. 2151 2152See Documentation/cachetlb.txt for more information on cache management. 2153 2154 2155CACHE COHERENCY VS MMIO 2156----------------------- 2157 2158Memory mapped I/O usually takes place through memory locations that are part of 2159a window in the CPU's memory space that has different properties assigned than 2160the usual RAM directed window. 2161 2162Amongst these properties is usually the fact that such accesses bypass the 2163caching entirely and go directly to the device buses. This means MMIO accesses 2164may, in effect, overtake accesses to cached memory that were emitted earlier. 2165A memory barrier isn't sufficient in such a case, but rather the cache must be 2166flushed between the cached memory write and the MMIO access if the two are in 2167any way dependent. 2168 2169 2170========================= 2171THE THINGS CPUS GET UP TO 2172========================= 2173 2174A programmer might take it for granted that the CPU will perform memory 2175operations in exactly the order specified, so that if the CPU is, for example, 2176given the following piece of code to execute: 2177 2178 a = *A; 2179 *B = b; 2180 c = *C; 2181 d = *D; 2182 *E = e; 2183 2184they would then expect that the CPU will complete the memory operation for each 2185instruction before moving on to the next one, leading to a definite sequence of 2186operations as seen by external observers in the system: 2187 2188 LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E. 2189 2190 2191Reality is, of course, much messier. With many CPUs and compilers, the above 2192assumption doesn't hold because: 2193 2194 (*) loads are more likely to need to be completed immediately to permit 2195 execution progress, whereas stores can often be deferred without a 2196 problem; 2197 2198 (*) loads may be done speculatively, and the result discarded should it prove 2199 to have been unnecessary; 2200 2201 (*) loads may be done speculatively, leading to the result having been fetched 2202 at the wrong time in the expected sequence of events; 2203 2204 (*) the order of the memory accesses may be rearranged to promote better use 2205 of the CPU buses and caches; 2206 2207 (*) loads and stores may be combined to improve performance when talking to 2208 memory or I/O hardware that can do batched accesses of adjacent locations, 2209 thus cutting down on transaction setup costs (memory and PCI devices may 2210 both be able to do this); and 2211 2212 (*) the CPU's data cache may affect the ordering, and whilst cache-coherency 2213 mechanisms may alleviate this - once the store has actually hit the cache 2214 - there's no guarantee that the coherency management will be propagated in 2215 order to other CPUs. 2216 2217So what another CPU, say, might actually observe from the above piece of code 2218is: 2219 2220 LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B 2221 2222 (Where "LOAD {*C,*D}" is a combined load) 2223 2224 2225However, it is guaranteed that a CPU will be self-consistent: it will see its 2226_own_ accesses appear to be correctly ordered, without the need for a memory 2227barrier. For instance with the following code: 2228 2229 U = *A; 2230 *A = V; 2231 *A = W; 2232 X = *A; 2233 *A = Y; 2234 Z = *A; 2235 2236and assuming no intervention by an external influence, it can be assumed that 2237the final result will appear to be: 2238 2239 U == the original value of *A 2240 X == W 2241 Z == Y 2242 *A == Y 2243 2244The code above may cause the CPU to generate the full sequence of memory 2245accesses: 2246 2247 U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A 2248 2249in that order, but, without intervention, the sequence may have almost any 2250combination of elements combined or discarded, provided the program's view of 2251the world remains consistent. 2252 2253The compiler may also combine, discard or defer elements of the sequence before 2254the CPU even sees them. 2255 2256For instance: 2257 2258 *A = V; 2259 *A = W; 2260 2261may be reduced to: 2262 2263 *A = W; 2264 2265since, without a write barrier, it can be assumed that the effect of the 2266storage of V to *A is lost. Similarly: 2267 2268 *A = Y; 2269 Z = *A; 2270 2271may, without a memory barrier, be reduced to: 2272 2273 *A = Y; 2274 Z = Y; 2275 2276and the LOAD operation never appear outside of the CPU. 2277 2278 2279AND THEN THERE'S THE ALPHA 2280-------------------------- 2281 2282The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that, 2283some versions of the Alpha CPU have a split data cache, permitting them to have 2284two semantically-related cache lines updated at separate times. This is where 2285the data dependency barrier really becomes necessary as this synchronises both 2286caches with the memory coherence system, thus making it seem like pointer 2287changes vs new data occur in the right order. 2288 2289The Alpha defines the Linux kernel's memory barrier model. 2290 2291See the subsection on "Cache Coherency" above. 2292 2293 2294============ 2295EXAMPLE USES 2296============ 2297 2298CIRCULAR BUFFERS 2299---------------- 2300 2301Memory barriers can be used to implement circular buffering without the need 2302of a lock to serialise the producer with the consumer. See: 2303 2304 Documentation/circular-buffers.txt 2305 2306for details. 2307 2308 2309========== 2310REFERENCES 2311========== 2312 2313Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek, 2314Digital Press) 2315 Chapter 5.2: Physical Address Space Characteristics 2316 Chapter 5.4: Caches and Write Buffers 2317 Chapter 5.5: Data Sharing 2318 Chapter 5.6: Read/Write Ordering 2319 2320AMD64 Architecture Programmer's Manual Volume 2: System Programming 2321 Chapter 7.1: Memory-Access Ordering 2322 Chapter 7.4: Buffering and Combining Memory Writes 2323 2324IA-32 Intel Architecture Software Developer's Manual, Volume 3: 2325System Programming Guide 2326 Chapter 7.1: Locked Atomic Operations 2327 Chapter 7.2: Memory Ordering 2328 Chapter 7.4: Serializing Instructions 2329 2330The SPARC Architecture Manual, Version 9 2331 Chapter 8: Memory Models 2332 Appendix D: Formal Specification of the Memory Models 2333 Appendix J: Programming with the Memory Models 2334 2335UltraSPARC Programmer Reference Manual 2336 Chapter 5: Memory Accesses and Cacheability 2337 Chapter 15: Sparc-V9 Memory Models 2338 2339UltraSPARC III Cu User's Manual 2340 Chapter 9: Memory Models 2341 2342UltraSPARC IIIi Processor User's Manual 2343 Chapter 8: Memory Models 2344 2345UltraSPARC Architecture 2005 2346 Chapter 9: Memory 2347 Appendix D: Formal Specifications of the Memory Models 2348 2349UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005 2350 Chapter 8: Memory Models 2351 Appendix F: Caches and Cache Coherency 2352 2353Solaris Internals, Core Kernel Architecture, p63-68: 2354 Chapter 3.3: Hardware Considerations for Locks and 2355 Synchronization 2356 2357Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching 2358for Kernel Programmers: 2359 Chapter 13: Other Memory Models 2360 2361Intel Itanium Architecture Software Developer's Manual: Volume 1: 2362 Section 2.6: Speculation 2363 Section 4.4: Memory Access 2364