1 /* 2 * Read-Copy Update mechanism for mutual exclusion 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License as published by 6 * the Free Software Foundation; either version 2 of the License, or 7 * (at your option) any later version. 8 * 9 * This program is distributed in the hope that it will be useful, 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 * GNU General Public License for more details. 13 * 14 * You should have received a copy of the GNU General Public License 15 * along with this program; if not, you can access it online at 16 * http://www.gnu.org/licenses/gpl-2.0.html. 17 * 18 * Copyright IBM Corporation, 2001 19 * 20 * Authors: Dipankar Sarma <dipankar@in.ibm.com> 21 * Manfred Spraul <manfred@colorfullife.com> 22 * 23 * Based on the original work by Paul McKenney <paulmck@us.ibm.com> 24 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen. 25 * Papers: 26 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf 27 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001) 28 * 29 * For detailed explanation of Read-Copy Update mechanism see - 30 * http://lse.sourceforge.net/locking/rcupdate.html 31 * 32 */ 33 #include <linux/types.h> 34 #include <linux/kernel.h> 35 #include <linux/init.h> 36 #include <linux/spinlock.h> 37 #include <linux/smp.h> 38 #include <linux/interrupt.h> 39 #include <linux/sched/signal.h> 40 #include <linux/sched/debug.h> 41 #include <linux/atomic.h> 42 #include <linux/bitops.h> 43 #include <linux/percpu.h> 44 #include <linux/notifier.h> 45 #include <linux/cpu.h> 46 #include <linux/mutex.h> 47 #include <linux/export.h> 48 #include <linux/hardirq.h> 49 #include <linux/delay.h> 50 #include <linux/moduleparam.h> 51 #include <linux/kthread.h> 52 #include <linux/tick.h> 53 #include <linux/rcupdate_wait.h> 54 55 #define CREATE_TRACE_POINTS 56 57 #include "rcu.h" 58 59 #ifdef MODULE_PARAM_PREFIX 60 #undef MODULE_PARAM_PREFIX 61 #endif 62 #define MODULE_PARAM_PREFIX "rcupdate." 63 64 #ifndef CONFIG_TINY_RCU 65 extern int rcu_expedited; /* from sysctl */ 66 module_param(rcu_expedited, int, 0); 67 extern int rcu_normal; /* from sysctl */ 68 module_param(rcu_normal, int, 0); 69 static int rcu_normal_after_boot; 70 module_param(rcu_normal_after_boot, int, 0); 71 #endif /* #ifndef CONFIG_TINY_RCU */ 72 73 #ifdef CONFIG_DEBUG_LOCK_ALLOC 74 /** 75 * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section? 76 * 77 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an 78 * RCU-sched read-side critical section. In absence of 79 * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side 80 * critical section unless it can prove otherwise. Note that disabling 81 * of preemption (including disabling irqs) counts as an RCU-sched 82 * read-side critical section. This is useful for debug checks in functions 83 * that required that they be called within an RCU-sched read-side 84 * critical section. 85 * 86 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot 87 * and while lockdep is disabled. 88 * 89 * Note that if the CPU is in the idle loop from an RCU point of 90 * view (ie: that we are in the section between rcu_idle_enter() and 91 * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU 92 * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs 93 * that are in such a section, considering these as in extended quiescent 94 * state, so such a CPU is effectively never in an RCU read-side critical 95 * section regardless of what RCU primitives it invokes. This state of 96 * affairs is required --- we need to keep an RCU-free window in idle 97 * where the CPU may possibly enter into low power mode. This way we can 98 * notice an extended quiescent state to other CPUs that started a grace 99 * period. Otherwise we would delay any grace period as long as we run in 100 * the idle task. 101 * 102 * Similarly, we avoid claiming an SRCU read lock held if the current 103 * CPU is offline. 104 */ 105 int rcu_read_lock_sched_held(void) 106 { 107 int lockdep_opinion = 0; 108 109 if (!debug_lockdep_rcu_enabled()) 110 return 1; 111 if (!rcu_is_watching()) 112 return 0; 113 if (!rcu_lockdep_current_cpu_online()) 114 return 0; 115 if (debug_locks) 116 lockdep_opinion = lock_is_held(&rcu_sched_lock_map); 117 return lockdep_opinion || !preemptible(); 118 } 119 EXPORT_SYMBOL(rcu_read_lock_sched_held); 120 #endif 121 122 #ifndef CONFIG_TINY_RCU 123 124 /* 125 * Should expedited grace-period primitives always fall back to their 126 * non-expedited counterparts? Intended for use within RCU. Note 127 * that if the user specifies both rcu_expedited and rcu_normal, then 128 * rcu_normal wins. (Except during the time period during boot from 129 * when the first task is spawned until the rcu_set_runtime_mode() 130 * core_initcall() is invoked, at which point everything is expedited.) 131 */ 132 bool rcu_gp_is_normal(void) 133 { 134 return READ_ONCE(rcu_normal) && 135 rcu_scheduler_active != RCU_SCHEDULER_INIT; 136 } 137 EXPORT_SYMBOL_GPL(rcu_gp_is_normal); 138 139 static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1); 140 141 /* 142 * Should normal grace-period primitives be expedited? Intended for 143 * use within RCU. Note that this function takes the rcu_expedited 144 * sysfs/boot variable and rcu_scheduler_active into account as well 145 * as the rcu_expedite_gp() nesting. So looping on rcu_unexpedite_gp() 146 * until rcu_gp_is_expedited() returns false is a -really- bad idea. 147 */ 148 bool rcu_gp_is_expedited(void) 149 { 150 return rcu_expedited || atomic_read(&rcu_expedited_nesting) || 151 rcu_scheduler_active == RCU_SCHEDULER_INIT; 152 } 153 EXPORT_SYMBOL_GPL(rcu_gp_is_expedited); 154 155 /** 156 * rcu_expedite_gp - Expedite future RCU grace periods 157 * 158 * After a call to this function, future calls to synchronize_rcu() and 159 * friends act as the corresponding synchronize_rcu_expedited() function 160 * had instead been called. 161 */ 162 void rcu_expedite_gp(void) 163 { 164 atomic_inc(&rcu_expedited_nesting); 165 } 166 EXPORT_SYMBOL_GPL(rcu_expedite_gp); 167 168 /** 169 * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation 170 * 171 * Undo a prior call to rcu_expedite_gp(). If all prior calls to 172 * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(), 173 * and if the rcu_expedited sysfs/boot parameter is not set, then all 174 * subsequent calls to synchronize_rcu() and friends will return to 175 * their normal non-expedited behavior. 176 */ 177 void rcu_unexpedite_gp(void) 178 { 179 atomic_dec(&rcu_expedited_nesting); 180 } 181 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp); 182 183 /* 184 * Inform RCU of the end of the in-kernel boot sequence. 185 */ 186 void rcu_end_inkernel_boot(void) 187 { 188 rcu_unexpedite_gp(); 189 if (rcu_normal_after_boot) 190 WRITE_ONCE(rcu_normal, 1); 191 } 192 193 #endif /* #ifndef CONFIG_TINY_RCU */ 194 195 /* 196 * Test each non-SRCU synchronous grace-period wait API. This is 197 * useful just after a change in mode for these primitives, and 198 * during early boot. 199 */ 200 void rcu_test_sync_prims(void) 201 { 202 if (!IS_ENABLED(CONFIG_PROVE_RCU)) 203 return; 204 synchronize_rcu(); 205 synchronize_rcu_bh(); 206 synchronize_sched(); 207 synchronize_rcu_expedited(); 208 synchronize_rcu_bh_expedited(); 209 synchronize_sched_expedited(); 210 } 211 212 #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) 213 214 /* 215 * Switch to run-time mode once RCU has fully initialized. 216 */ 217 static int __init rcu_set_runtime_mode(void) 218 { 219 rcu_test_sync_prims(); 220 rcu_scheduler_active = RCU_SCHEDULER_RUNNING; 221 rcu_test_sync_prims(); 222 return 0; 223 } 224 core_initcall(rcu_set_runtime_mode); 225 226 #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */ 227 228 #ifdef CONFIG_PREEMPT_RCU 229 230 /* 231 * Preemptible RCU implementation for rcu_read_lock(). 232 * Just increment ->rcu_read_lock_nesting, shared state will be updated 233 * if we block. 234 */ 235 void __rcu_read_lock(void) 236 { 237 current->rcu_read_lock_nesting++; 238 barrier(); /* critical section after entry code. */ 239 } 240 EXPORT_SYMBOL_GPL(__rcu_read_lock); 241 242 /* 243 * Preemptible RCU implementation for rcu_read_unlock(). 244 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost 245 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then 246 * invoke rcu_read_unlock_special() to clean up after a context switch 247 * in an RCU read-side critical section and other special cases. 248 */ 249 void __rcu_read_unlock(void) 250 { 251 struct task_struct *t = current; 252 253 if (t->rcu_read_lock_nesting != 1) { 254 --t->rcu_read_lock_nesting; 255 } else { 256 barrier(); /* critical section before exit code. */ 257 t->rcu_read_lock_nesting = INT_MIN; 258 barrier(); /* assign before ->rcu_read_unlock_special load */ 259 if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) 260 rcu_read_unlock_special(t); 261 barrier(); /* ->rcu_read_unlock_special load before assign */ 262 t->rcu_read_lock_nesting = 0; 263 } 264 #ifdef CONFIG_PROVE_LOCKING 265 { 266 int rrln = READ_ONCE(t->rcu_read_lock_nesting); 267 268 WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2); 269 } 270 #endif /* #ifdef CONFIG_PROVE_LOCKING */ 271 } 272 EXPORT_SYMBOL_GPL(__rcu_read_unlock); 273 274 #endif /* #ifdef CONFIG_PREEMPT_RCU */ 275 276 #ifdef CONFIG_DEBUG_LOCK_ALLOC 277 static struct lock_class_key rcu_lock_key; 278 struct lockdep_map rcu_lock_map = 279 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key); 280 EXPORT_SYMBOL_GPL(rcu_lock_map); 281 282 static struct lock_class_key rcu_bh_lock_key; 283 struct lockdep_map rcu_bh_lock_map = 284 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key); 285 EXPORT_SYMBOL_GPL(rcu_bh_lock_map); 286 287 static struct lock_class_key rcu_sched_lock_key; 288 struct lockdep_map rcu_sched_lock_map = 289 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key); 290 EXPORT_SYMBOL_GPL(rcu_sched_lock_map); 291 292 static struct lock_class_key rcu_callback_key; 293 struct lockdep_map rcu_callback_map = 294 STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key); 295 EXPORT_SYMBOL_GPL(rcu_callback_map); 296 297 int notrace debug_lockdep_rcu_enabled(void) 298 { 299 return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && 300 current->lockdep_recursion == 0; 301 } 302 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled); 303 304 /** 305 * rcu_read_lock_held() - might we be in RCU read-side critical section? 306 * 307 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU 308 * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC, 309 * this assumes we are in an RCU read-side critical section unless it can 310 * prove otherwise. This is useful for debug checks in functions that 311 * require that they be called within an RCU read-side critical section. 312 * 313 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot 314 * and while lockdep is disabled. 315 * 316 * Note that rcu_read_lock() and the matching rcu_read_unlock() must 317 * occur in the same context, for example, it is illegal to invoke 318 * rcu_read_unlock() in process context if the matching rcu_read_lock() 319 * was invoked from within an irq handler. 320 * 321 * Note that rcu_read_lock() is disallowed if the CPU is either idle or 322 * offline from an RCU perspective, so check for those as well. 323 */ 324 int rcu_read_lock_held(void) 325 { 326 if (!debug_lockdep_rcu_enabled()) 327 return 1; 328 if (!rcu_is_watching()) 329 return 0; 330 if (!rcu_lockdep_current_cpu_online()) 331 return 0; 332 return lock_is_held(&rcu_lock_map); 333 } 334 EXPORT_SYMBOL_GPL(rcu_read_lock_held); 335 336 /** 337 * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section? 338 * 339 * Check for bottom half being disabled, which covers both the 340 * CONFIG_PROVE_RCU and not cases. Note that if someone uses 341 * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled) 342 * will show the situation. This is useful for debug checks in functions 343 * that require that they be called within an RCU read-side critical 344 * section. 345 * 346 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot. 347 * 348 * Note that rcu_read_lock() is disallowed if the CPU is either idle or 349 * offline from an RCU perspective, so check for those as well. 350 */ 351 int rcu_read_lock_bh_held(void) 352 { 353 if (!debug_lockdep_rcu_enabled()) 354 return 1; 355 if (!rcu_is_watching()) 356 return 0; 357 if (!rcu_lockdep_current_cpu_online()) 358 return 0; 359 return in_softirq() || irqs_disabled(); 360 } 361 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held); 362 363 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ 364 365 /** 366 * wakeme_after_rcu() - Callback function to awaken a task after grace period 367 * @head: Pointer to rcu_head member within rcu_synchronize structure 368 * 369 * Awaken the corresponding task now that a grace period has elapsed. 370 */ 371 void wakeme_after_rcu(struct rcu_head *head) 372 { 373 struct rcu_synchronize *rcu; 374 375 rcu = container_of(head, struct rcu_synchronize, head); 376 complete(&rcu->completion); 377 } 378 EXPORT_SYMBOL_GPL(wakeme_after_rcu); 379 380 void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array, 381 struct rcu_synchronize *rs_array) 382 { 383 int i; 384 int j; 385 386 /* Initialize and register callbacks for each flavor specified. */ 387 for (i = 0; i < n; i++) { 388 if (checktiny && 389 (crcu_array[i] == call_rcu || 390 crcu_array[i] == call_rcu_bh)) { 391 might_sleep(); 392 continue; 393 } 394 init_rcu_head_on_stack(&rs_array[i].head); 395 init_completion(&rs_array[i].completion); 396 for (j = 0; j < i; j++) 397 if (crcu_array[j] == crcu_array[i]) 398 break; 399 if (j == i) 400 (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu); 401 } 402 403 /* Wait for all callbacks to be invoked. */ 404 for (i = 0; i < n; i++) { 405 if (checktiny && 406 (crcu_array[i] == call_rcu || 407 crcu_array[i] == call_rcu_bh)) 408 continue; 409 for (j = 0; j < i; j++) 410 if (crcu_array[j] == crcu_array[i]) 411 break; 412 if (j == i) 413 wait_for_completion(&rs_array[i].completion); 414 destroy_rcu_head_on_stack(&rs_array[i].head); 415 } 416 } 417 EXPORT_SYMBOL_GPL(__wait_rcu_gp); 418 419 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD 420 void init_rcu_head(struct rcu_head *head) 421 { 422 debug_object_init(head, &rcuhead_debug_descr); 423 } 424 425 void destroy_rcu_head(struct rcu_head *head) 426 { 427 debug_object_free(head, &rcuhead_debug_descr); 428 } 429 430 static bool rcuhead_is_static_object(void *addr) 431 { 432 return true; 433 } 434 435 /** 436 * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects 437 * @head: pointer to rcu_head structure to be initialized 438 * 439 * This function informs debugobjects of a new rcu_head structure that 440 * has been allocated as an auto variable on the stack. This function 441 * is not required for rcu_head structures that are statically defined or 442 * that are dynamically allocated on the heap. This function has no 443 * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. 444 */ 445 void init_rcu_head_on_stack(struct rcu_head *head) 446 { 447 debug_object_init_on_stack(head, &rcuhead_debug_descr); 448 } 449 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack); 450 451 /** 452 * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects 453 * @head: pointer to rcu_head structure to be initialized 454 * 455 * This function informs debugobjects that an on-stack rcu_head structure 456 * is about to go out of scope. As with init_rcu_head_on_stack(), this 457 * function is not required for rcu_head structures that are statically 458 * defined or that are dynamically allocated on the heap. Also as with 459 * init_rcu_head_on_stack(), this function has no effect for 460 * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. 461 */ 462 void destroy_rcu_head_on_stack(struct rcu_head *head) 463 { 464 debug_object_free(head, &rcuhead_debug_descr); 465 } 466 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack); 467 468 struct debug_obj_descr rcuhead_debug_descr = { 469 .name = "rcu_head", 470 .is_static_object = rcuhead_is_static_object, 471 }; 472 EXPORT_SYMBOL_GPL(rcuhead_debug_descr); 473 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */ 474 475 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE) 476 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp, 477 unsigned long secs, 478 unsigned long c_old, unsigned long c) 479 { 480 trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c); 481 } 482 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read); 483 #else 484 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \ 485 do { } while (0) 486 #endif 487 488 #ifdef CONFIG_RCU_STALL_COMMON 489 490 #ifdef CONFIG_PROVE_RCU 491 #define RCU_STALL_DELAY_DELTA (5 * HZ) 492 #else 493 #define RCU_STALL_DELAY_DELTA 0 494 #endif 495 496 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */ 497 static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT; 498 499 module_param(rcu_cpu_stall_suppress, int, 0644); 500 module_param(rcu_cpu_stall_timeout, int, 0644); 501 502 int rcu_jiffies_till_stall_check(void) 503 { 504 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout); 505 506 /* 507 * Limit check must be consistent with the Kconfig limits 508 * for CONFIG_RCU_CPU_STALL_TIMEOUT. 509 */ 510 if (till_stall_check < 3) { 511 WRITE_ONCE(rcu_cpu_stall_timeout, 3); 512 till_stall_check = 3; 513 } else if (till_stall_check > 300) { 514 WRITE_ONCE(rcu_cpu_stall_timeout, 300); 515 till_stall_check = 300; 516 } 517 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA; 518 } 519 520 void rcu_sysrq_start(void) 521 { 522 if (!rcu_cpu_stall_suppress) 523 rcu_cpu_stall_suppress = 2; 524 } 525 526 void rcu_sysrq_end(void) 527 { 528 if (rcu_cpu_stall_suppress == 2) 529 rcu_cpu_stall_suppress = 0; 530 } 531 532 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr) 533 { 534 rcu_cpu_stall_suppress = 1; 535 return NOTIFY_DONE; 536 } 537 538 static struct notifier_block rcu_panic_block = { 539 .notifier_call = rcu_panic, 540 }; 541 542 static int __init check_cpu_stall_init(void) 543 { 544 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block); 545 return 0; 546 } 547 early_initcall(check_cpu_stall_init); 548 549 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */ 550 551 #ifdef CONFIG_TASKS_RCU 552 553 /* 554 * Simple variant of RCU whose quiescent states are voluntary context switch, 555 * user-space execution, and idle. As such, grace periods can take one good 556 * long time. There are no read-side primitives similar to rcu_read_lock() 557 * and rcu_read_unlock() because this implementation is intended to get 558 * the system into a safe state for some of the manipulations involved in 559 * tracing and the like. Finally, this implementation does not support 560 * high call_rcu_tasks() rates from multiple CPUs. If this is required, 561 * per-CPU callback lists will be needed. 562 */ 563 564 /* Global list of callbacks and associated lock. */ 565 static struct rcu_head *rcu_tasks_cbs_head; 566 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; 567 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq); 568 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock); 569 570 /* Track exiting tasks in order to allow them to be waited for. */ 571 DEFINE_SRCU(tasks_rcu_exit_srcu); 572 573 /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */ 574 #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10) 575 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT; 576 module_param(rcu_task_stall_timeout, int, 0644); 577 578 static void rcu_spawn_tasks_kthread(void); 579 static struct task_struct *rcu_tasks_kthread_ptr; 580 581 /** 582 * call_rcu_tasks() - Queue an RCU for invocation task-based grace period 583 * @rhp: structure to be used for queueing the RCU updates. 584 * @func: actual callback function to be invoked after the grace period 585 * 586 * The callback function will be invoked some time after a full grace 587 * period elapses, in other words after all currently executing RCU 588 * read-side critical sections have completed. call_rcu_tasks() assumes 589 * that the read-side critical sections end at a voluntary context 590 * switch (not a preemption!), entry into idle, or transition to usermode 591 * execution. As such, there are no read-side primitives analogous to 592 * rcu_read_lock() and rcu_read_unlock() because this primitive is intended 593 * to determine that all tasks have passed through a safe state, not so 594 * much for data-strcuture synchronization. 595 * 596 * See the description of call_rcu() for more detailed information on 597 * memory ordering guarantees. 598 */ 599 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func) 600 { 601 unsigned long flags; 602 bool needwake; 603 bool havetask = READ_ONCE(rcu_tasks_kthread_ptr); 604 605 rhp->next = NULL; 606 rhp->func = func; 607 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); 608 needwake = !rcu_tasks_cbs_head; 609 *rcu_tasks_cbs_tail = rhp; 610 rcu_tasks_cbs_tail = &rhp->next; 611 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); 612 /* We can't create the thread unless interrupts are enabled. */ 613 if ((needwake && havetask) || 614 (!havetask && !irqs_disabled_flags(flags))) { 615 rcu_spawn_tasks_kthread(); 616 wake_up(&rcu_tasks_cbs_wq); 617 } 618 } 619 EXPORT_SYMBOL_GPL(call_rcu_tasks); 620 621 /** 622 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed. 623 * 624 * Control will return to the caller some time after a full rcu-tasks 625 * grace period has elapsed, in other words after all currently 626 * executing rcu-tasks read-side critical sections have elapsed. These 627 * read-side critical sections are delimited by calls to schedule(), 628 * cond_resched_rcu_qs(), idle execution, userspace execution, calls 629 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched(). 630 * 631 * This is a very specialized primitive, intended only for a few uses in 632 * tracing and other situations requiring manipulation of function 633 * preambles and profiling hooks. The synchronize_rcu_tasks() function 634 * is not (yet) intended for heavy use from multiple CPUs. 635 * 636 * Note that this guarantee implies further memory-ordering guarantees. 637 * On systems with more than one CPU, when synchronize_rcu_tasks() returns, 638 * each CPU is guaranteed to have executed a full memory barrier since the 639 * end of its last RCU-tasks read-side critical section whose beginning 640 * preceded the call to synchronize_rcu_tasks(). In addition, each CPU 641 * having an RCU-tasks read-side critical section that extends beyond 642 * the return from synchronize_rcu_tasks() is guaranteed to have executed 643 * a full memory barrier after the beginning of synchronize_rcu_tasks() 644 * and before the beginning of that RCU-tasks read-side critical section. 645 * Note that these guarantees include CPUs that are offline, idle, or 646 * executing in user mode, as well as CPUs that are executing in the kernel. 647 * 648 * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned 649 * to its caller on CPU B, then both CPU A and CPU B are guaranteed 650 * to have executed a full memory barrier during the execution of 651 * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU 652 * (but again only if the system has more than one CPU). 653 */ 654 void synchronize_rcu_tasks(void) 655 { 656 /* Complain if the scheduler has not started. */ 657 RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE, 658 "synchronize_rcu_tasks called too soon"); 659 660 /* Wait for the grace period. */ 661 wait_rcu_gp(call_rcu_tasks); 662 } 663 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks); 664 665 /** 666 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks. 667 * 668 * Although the current implementation is guaranteed to wait, it is not 669 * obligated to, for example, if there are no pending callbacks. 670 */ 671 void rcu_barrier_tasks(void) 672 { 673 /* There is only one callback queue, so this is easy. ;-) */ 674 synchronize_rcu_tasks(); 675 } 676 EXPORT_SYMBOL_GPL(rcu_barrier_tasks); 677 678 /* See if tasks are still holding out, complain if so. */ 679 static void check_holdout_task(struct task_struct *t, 680 bool needreport, bool *firstreport) 681 { 682 int cpu; 683 684 if (!READ_ONCE(t->rcu_tasks_holdout) || 685 t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) || 686 !READ_ONCE(t->on_rq) || 687 (IS_ENABLED(CONFIG_NO_HZ_FULL) && 688 !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) { 689 WRITE_ONCE(t->rcu_tasks_holdout, false); 690 list_del_init(&t->rcu_tasks_holdout_list); 691 put_task_struct(t); 692 return; 693 } 694 rcu_request_urgent_qs_task(t); 695 if (!needreport) 696 return; 697 if (*firstreport) { 698 pr_err("INFO: rcu_tasks detected stalls on tasks:\n"); 699 *firstreport = false; 700 } 701 cpu = task_cpu(t); 702 pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n", 703 t, ".I"[is_idle_task(t)], 704 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)], 705 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout, 706 t->rcu_tasks_idle_cpu, cpu); 707 sched_show_task(t); 708 } 709 710 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */ 711 static int __noreturn rcu_tasks_kthread(void *arg) 712 { 713 unsigned long flags; 714 struct task_struct *g, *t; 715 unsigned long lastreport; 716 struct rcu_head *list; 717 struct rcu_head *next; 718 LIST_HEAD(rcu_tasks_holdouts); 719 720 /* Run on housekeeping CPUs by default. Sysadm can move if desired. */ 721 housekeeping_affine(current); 722 723 /* 724 * Each pass through the following loop makes one check for 725 * newly arrived callbacks, and, if there are some, waits for 726 * one RCU-tasks grace period and then invokes the callbacks. 727 * This loop is terminated by the system going down. ;-) 728 */ 729 for (;;) { 730 731 /* Pick up any new callbacks. */ 732 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); 733 list = rcu_tasks_cbs_head; 734 rcu_tasks_cbs_head = NULL; 735 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; 736 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); 737 738 /* If there were none, wait a bit and start over. */ 739 if (!list) { 740 wait_event_interruptible(rcu_tasks_cbs_wq, 741 rcu_tasks_cbs_head); 742 if (!rcu_tasks_cbs_head) { 743 WARN_ON(signal_pending(current)); 744 schedule_timeout_interruptible(HZ/10); 745 } 746 continue; 747 } 748 749 /* 750 * Wait for all pre-existing t->on_rq and t->nvcsw 751 * transitions to complete. Invoking synchronize_sched() 752 * suffices because all these transitions occur with 753 * interrupts disabled. Without this synchronize_sched(), 754 * a read-side critical section that started before the 755 * grace period might be incorrectly seen as having started 756 * after the grace period. 757 * 758 * This synchronize_sched() also dispenses with the 759 * need for a memory barrier on the first store to 760 * ->rcu_tasks_holdout, as it forces the store to happen 761 * after the beginning of the grace period. 762 */ 763 synchronize_sched(); 764 765 /* 766 * There were callbacks, so we need to wait for an 767 * RCU-tasks grace period. Start off by scanning 768 * the task list for tasks that are not already 769 * voluntarily blocked. Mark these tasks and make 770 * a list of them in rcu_tasks_holdouts. 771 */ 772 rcu_read_lock(); 773 for_each_process_thread(g, t) { 774 if (t != current && READ_ONCE(t->on_rq) && 775 !is_idle_task(t)) { 776 get_task_struct(t); 777 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw); 778 WRITE_ONCE(t->rcu_tasks_holdout, true); 779 list_add(&t->rcu_tasks_holdout_list, 780 &rcu_tasks_holdouts); 781 } 782 } 783 rcu_read_unlock(); 784 785 /* 786 * Wait for tasks that are in the process of exiting. 787 * This does only part of the job, ensuring that all 788 * tasks that were previously exiting reach the point 789 * where they have disabled preemption, allowing the 790 * later synchronize_sched() to finish the job. 791 */ 792 synchronize_srcu(&tasks_rcu_exit_srcu); 793 794 /* 795 * Each pass through the following loop scans the list 796 * of holdout tasks, removing any that are no longer 797 * holdouts. When the list is empty, we are done. 798 */ 799 lastreport = jiffies; 800 while (!list_empty(&rcu_tasks_holdouts)) { 801 bool firstreport; 802 bool needreport; 803 int rtst; 804 struct task_struct *t1; 805 806 schedule_timeout_interruptible(HZ); 807 rtst = READ_ONCE(rcu_task_stall_timeout); 808 needreport = rtst > 0 && 809 time_after(jiffies, lastreport + rtst); 810 if (needreport) 811 lastreport = jiffies; 812 firstreport = true; 813 WARN_ON(signal_pending(current)); 814 list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts, 815 rcu_tasks_holdout_list) { 816 check_holdout_task(t, needreport, &firstreport); 817 cond_resched(); 818 } 819 } 820 821 /* 822 * Because ->on_rq and ->nvcsw are not guaranteed 823 * to have a full memory barriers prior to them in the 824 * schedule() path, memory reordering on other CPUs could 825 * cause their RCU-tasks read-side critical sections to 826 * extend past the end of the grace period. However, 827 * because these ->nvcsw updates are carried out with 828 * interrupts disabled, we can use synchronize_sched() 829 * to force the needed ordering on all such CPUs. 830 * 831 * This synchronize_sched() also confines all 832 * ->rcu_tasks_holdout accesses to be within the grace 833 * period, avoiding the need for memory barriers for 834 * ->rcu_tasks_holdout accesses. 835 * 836 * In addition, this synchronize_sched() waits for exiting 837 * tasks to complete their final preempt_disable() region 838 * of execution, cleaning up after the synchronize_srcu() 839 * above. 840 */ 841 synchronize_sched(); 842 843 /* Invoke the callbacks. */ 844 while (list) { 845 next = list->next; 846 local_bh_disable(); 847 list->func(list); 848 local_bh_enable(); 849 list = next; 850 cond_resched(); 851 } 852 schedule_timeout_uninterruptible(HZ/10); 853 } 854 } 855 856 /* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */ 857 static void rcu_spawn_tasks_kthread(void) 858 { 859 static DEFINE_MUTEX(rcu_tasks_kthread_mutex); 860 struct task_struct *t; 861 862 if (READ_ONCE(rcu_tasks_kthread_ptr)) { 863 smp_mb(); /* Ensure caller sees full kthread. */ 864 return; 865 } 866 mutex_lock(&rcu_tasks_kthread_mutex); 867 if (rcu_tasks_kthread_ptr) { 868 mutex_unlock(&rcu_tasks_kthread_mutex); 869 return; 870 } 871 t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread"); 872 BUG_ON(IS_ERR(t)); 873 smp_mb(); /* Ensure others see full kthread. */ 874 WRITE_ONCE(rcu_tasks_kthread_ptr, t); 875 mutex_unlock(&rcu_tasks_kthread_mutex); 876 } 877 878 #endif /* #ifdef CONFIG_TASKS_RCU */ 879 880 #ifndef CONFIG_TINY_RCU 881 882 /* 883 * Print any non-default Tasks RCU settings. 884 */ 885 static void __init rcu_tasks_bootup_oddness(void) 886 { 887 #ifdef CONFIG_TASKS_RCU 888 if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT) 889 pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout); 890 else 891 pr_info("\tTasks RCU enabled.\n"); 892 #endif /* #ifdef CONFIG_TASKS_RCU */ 893 } 894 895 #endif /* #ifndef CONFIG_TINY_RCU */ 896 897 #ifdef CONFIG_PROVE_RCU 898 899 /* 900 * Early boot self test parameters, one for each flavor 901 */ 902 static bool rcu_self_test; 903 static bool rcu_self_test_bh; 904 static bool rcu_self_test_sched; 905 906 module_param(rcu_self_test, bool, 0444); 907 module_param(rcu_self_test_bh, bool, 0444); 908 module_param(rcu_self_test_sched, bool, 0444); 909 910 static int rcu_self_test_counter; 911 912 static void test_callback(struct rcu_head *r) 913 { 914 rcu_self_test_counter++; 915 pr_info("RCU test callback executed %d\n", rcu_self_test_counter); 916 } 917 918 static void early_boot_test_call_rcu(void) 919 { 920 static struct rcu_head head; 921 922 call_rcu(&head, test_callback); 923 } 924 925 static void early_boot_test_call_rcu_bh(void) 926 { 927 static struct rcu_head head; 928 929 call_rcu_bh(&head, test_callback); 930 } 931 932 static void early_boot_test_call_rcu_sched(void) 933 { 934 static struct rcu_head head; 935 936 call_rcu_sched(&head, test_callback); 937 } 938 939 void rcu_early_boot_tests(void) 940 { 941 pr_info("Running RCU self tests\n"); 942 943 if (rcu_self_test) 944 early_boot_test_call_rcu(); 945 if (rcu_self_test_bh) 946 early_boot_test_call_rcu_bh(); 947 if (rcu_self_test_sched) 948 early_boot_test_call_rcu_sched(); 949 rcu_test_sync_prims(); 950 } 951 952 static int rcu_verify_early_boot_tests(void) 953 { 954 int ret = 0; 955 int early_boot_test_counter = 0; 956 957 if (rcu_self_test) { 958 early_boot_test_counter++; 959 rcu_barrier(); 960 } 961 if (rcu_self_test_bh) { 962 early_boot_test_counter++; 963 rcu_barrier_bh(); 964 } 965 if (rcu_self_test_sched) { 966 early_boot_test_counter++; 967 rcu_barrier_sched(); 968 } 969 970 if (rcu_self_test_counter != early_boot_test_counter) { 971 WARN_ON(1); 972 ret = -1; 973 } 974 975 return ret; 976 } 977 late_initcall(rcu_verify_early_boot_tests); 978 #else 979 void rcu_early_boot_tests(void) {} 980 #endif /* CONFIG_PROVE_RCU */ 981 982 #ifndef CONFIG_TINY_RCU 983 984 /* 985 * Print any significant non-default boot-time settings. 986 */ 987 void __init rcupdate_announce_bootup_oddness(void) 988 { 989 if (rcu_normal) 990 pr_info("\tNo expedited grace period (rcu_normal).\n"); 991 else if (rcu_normal_after_boot) 992 pr_info("\tNo expedited grace period (rcu_normal_after_boot).\n"); 993 else if (rcu_expedited) 994 pr_info("\tAll grace periods are expedited (rcu_expedited).\n"); 995 if (rcu_cpu_stall_suppress) 996 pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n"); 997 if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT) 998 pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout); 999 rcu_tasks_bootup_oddness(); 1000 } 1001 1002 #endif /* #ifndef CONFIG_TINY_RCU */ 1003