1 /* 2 * RCU expedited grace periods 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, 2016 19 * 20 * Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com> 21 */ 22 23 /* Wrapper functions for expedited grace periods. */ 24 static void rcu_exp_gp_seq_start(struct rcu_state *rsp) 25 { 26 rcu_seq_start(&rsp->expedited_sequence); 27 } 28 static void rcu_exp_gp_seq_end(struct rcu_state *rsp) 29 { 30 rcu_seq_end(&rsp->expedited_sequence); 31 smp_mb(); /* Ensure that consecutive grace periods serialize. */ 32 } 33 static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp) 34 { 35 unsigned long s; 36 37 smp_mb(); /* Caller's modifications seen first by other CPUs. */ 38 s = rcu_seq_snap(&rsp->expedited_sequence); 39 trace_rcu_exp_grace_period(rsp->name, s, TPS("snap")); 40 return s; 41 } 42 static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s) 43 { 44 return rcu_seq_done(&rsp->expedited_sequence, s); 45 } 46 47 /* 48 * Reset the ->expmaskinit values in the rcu_node tree to reflect any 49 * recent CPU-online activity. Note that these masks are not cleared 50 * when CPUs go offline, so they reflect the union of all CPUs that have 51 * ever been online. This means that this function normally takes its 52 * no-work-to-do fastpath. 53 */ 54 static void sync_exp_reset_tree_hotplug(struct rcu_state *rsp) 55 { 56 bool done; 57 unsigned long flags; 58 unsigned long mask; 59 unsigned long oldmask; 60 int ncpus = READ_ONCE(rsp->ncpus); 61 struct rcu_node *rnp; 62 struct rcu_node *rnp_up; 63 64 /* If no new CPUs onlined since last time, nothing to do. */ 65 if (likely(ncpus == rsp->ncpus_snap)) 66 return; 67 rsp->ncpus_snap = ncpus; 68 69 /* 70 * Each pass through the following loop propagates newly onlined 71 * CPUs for the current rcu_node structure up the rcu_node tree. 72 */ 73 rcu_for_each_leaf_node(rsp, rnp) { 74 raw_spin_lock_irqsave_rcu_node(rnp, flags); 75 if (rnp->expmaskinit == rnp->expmaskinitnext) { 76 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 77 continue; /* No new CPUs, nothing to do. */ 78 } 79 80 /* Update this node's mask, track old value for propagation. */ 81 oldmask = rnp->expmaskinit; 82 rnp->expmaskinit = rnp->expmaskinitnext; 83 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 84 85 /* If was already nonzero, nothing to propagate. */ 86 if (oldmask) 87 continue; 88 89 /* Propagate the new CPU up the tree. */ 90 mask = rnp->grpmask; 91 rnp_up = rnp->parent; 92 done = false; 93 while (rnp_up) { 94 raw_spin_lock_irqsave_rcu_node(rnp_up, flags); 95 if (rnp_up->expmaskinit) 96 done = true; 97 rnp_up->expmaskinit |= mask; 98 raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags); 99 if (done) 100 break; 101 mask = rnp_up->grpmask; 102 rnp_up = rnp_up->parent; 103 } 104 } 105 } 106 107 /* 108 * Reset the ->expmask values in the rcu_node tree in preparation for 109 * a new expedited grace period. 110 */ 111 static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp) 112 { 113 unsigned long flags; 114 struct rcu_node *rnp; 115 116 sync_exp_reset_tree_hotplug(rsp); 117 rcu_for_each_node_breadth_first(rsp, rnp) { 118 raw_spin_lock_irqsave_rcu_node(rnp, flags); 119 WARN_ON_ONCE(rnp->expmask); 120 rnp->expmask = rnp->expmaskinit; 121 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 122 } 123 } 124 125 /* 126 * Return non-zero if there is no RCU expedited grace period in progress 127 * for the specified rcu_node structure, in other words, if all CPUs and 128 * tasks covered by the specified rcu_node structure have done their bit 129 * for the current expedited grace period. Works only for preemptible 130 * RCU -- other RCU implementation use other means. 131 * 132 * Caller must hold the rcu_state's exp_mutex. 133 */ 134 static int sync_rcu_preempt_exp_done(struct rcu_node *rnp) 135 { 136 return rnp->exp_tasks == NULL && 137 READ_ONCE(rnp->expmask) == 0; 138 } 139 140 /* 141 * Report the exit from RCU read-side critical section for the last task 142 * that queued itself during or before the current expedited preemptible-RCU 143 * grace period. This event is reported either to the rcu_node structure on 144 * which the task was queued or to one of that rcu_node structure's ancestors, 145 * recursively up the tree. (Calm down, calm down, we do the recursion 146 * iteratively!) 147 * 148 * Caller must hold the rcu_state's exp_mutex and the specified rcu_node 149 * structure's ->lock. 150 */ 151 static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp, 152 bool wake, unsigned long flags) 153 __releases(rnp->lock) 154 { 155 unsigned long mask; 156 157 for (;;) { 158 if (!sync_rcu_preempt_exp_done(rnp)) { 159 if (!rnp->expmask) 160 rcu_initiate_boost(rnp, flags); 161 else 162 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 163 break; 164 } 165 if (rnp->parent == NULL) { 166 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 167 if (wake) { 168 smp_mb(); /* EGP done before wake_up(). */ 169 swake_up(&rsp->expedited_wq); 170 } 171 break; 172 } 173 mask = rnp->grpmask; 174 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */ 175 rnp = rnp->parent; 176 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */ 177 WARN_ON_ONCE(!(rnp->expmask & mask)); 178 rnp->expmask &= ~mask; 179 } 180 } 181 182 /* 183 * Report expedited quiescent state for specified node. This is a 184 * lock-acquisition wrapper function for __rcu_report_exp_rnp(). 185 * 186 * Caller must hold the rcu_state's exp_mutex. 187 */ 188 static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp, 189 struct rcu_node *rnp, bool wake) 190 { 191 unsigned long flags; 192 193 raw_spin_lock_irqsave_rcu_node(rnp, flags); 194 __rcu_report_exp_rnp(rsp, rnp, wake, flags); 195 } 196 197 /* 198 * Report expedited quiescent state for multiple CPUs, all covered by the 199 * specified leaf rcu_node structure. Caller must hold the rcu_state's 200 * exp_mutex. 201 */ 202 static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp, 203 unsigned long mask, bool wake) 204 { 205 unsigned long flags; 206 207 raw_spin_lock_irqsave_rcu_node(rnp, flags); 208 if (!(rnp->expmask & mask)) { 209 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 210 return; 211 } 212 rnp->expmask &= ~mask; 213 __rcu_report_exp_rnp(rsp, rnp, wake, flags); /* Releases rnp->lock. */ 214 } 215 216 /* 217 * Report expedited quiescent state for specified rcu_data (CPU). 218 */ 219 static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp, 220 bool wake) 221 { 222 rcu_report_exp_cpu_mult(rsp, rdp->mynode, rdp->grpmask, wake); 223 } 224 225 /* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */ 226 static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat, 227 unsigned long s) 228 { 229 if (rcu_exp_gp_seq_done(rsp, s)) { 230 trace_rcu_exp_grace_period(rsp->name, s, TPS("done")); 231 /* Ensure test happens before caller kfree(). */ 232 smp_mb__before_atomic(); /* ^^^ */ 233 atomic_long_inc(stat); 234 return true; 235 } 236 return false; 237 } 238 239 /* 240 * Funnel-lock acquisition for expedited grace periods. Returns true 241 * if some other task completed an expedited grace period that this task 242 * can piggy-back on, and with no mutex held. Otherwise, returns false 243 * with the mutex held, indicating that the caller must actually do the 244 * expedited grace period. 245 */ 246 static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s) 247 { 248 struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id()); 249 struct rcu_node *rnp = rdp->mynode; 250 struct rcu_node *rnp_root = rcu_get_root(rsp); 251 252 /* Low-contention fastpath. */ 253 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) && 254 (rnp == rnp_root || 255 ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) && 256 mutex_trylock(&rsp->exp_mutex)) 257 goto fastpath; 258 259 /* 260 * Each pass through the following loop works its way up 261 * the rcu_node tree, returning if others have done the work or 262 * otherwise falls through to acquire rsp->exp_mutex. The mapping 263 * from CPU to rcu_node structure can be inexact, as it is just 264 * promoting locality and is not strictly needed for correctness. 265 */ 266 for (; rnp != NULL; rnp = rnp->parent) { 267 if (sync_exp_work_done(rsp, &rdp->exp_workdone1, s)) 268 return true; 269 270 /* Work not done, either wait here or go up. */ 271 spin_lock(&rnp->exp_lock); 272 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) { 273 274 /* Someone else doing GP, so wait for them. */ 275 spin_unlock(&rnp->exp_lock); 276 trace_rcu_exp_funnel_lock(rsp->name, rnp->level, 277 rnp->grplo, rnp->grphi, 278 TPS("wait")); 279 wait_event(rnp->exp_wq[(s >> 1) & 0x3], 280 sync_exp_work_done(rsp, 281 &rdp->exp_workdone2, s)); 282 return true; 283 } 284 rnp->exp_seq_rq = s; /* Followers can wait on us. */ 285 spin_unlock(&rnp->exp_lock); 286 trace_rcu_exp_funnel_lock(rsp->name, rnp->level, rnp->grplo, 287 rnp->grphi, TPS("nxtlvl")); 288 } 289 mutex_lock(&rsp->exp_mutex); 290 fastpath: 291 if (sync_exp_work_done(rsp, &rdp->exp_workdone3, s)) { 292 mutex_unlock(&rsp->exp_mutex); 293 return true; 294 } 295 rcu_exp_gp_seq_start(rsp); 296 trace_rcu_exp_grace_period(rsp->name, s, TPS("start")); 297 return false; 298 } 299 300 /* Invoked on each online non-idle CPU for expedited quiescent state. */ 301 static void sync_sched_exp_handler(void *data) 302 { 303 struct rcu_data *rdp; 304 struct rcu_node *rnp; 305 struct rcu_state *rsp = data; 306 307 rdp = this_cpu_ptr(rsp->rda); 308 rnp = rdp->mynode; 309 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) || 310 __this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp)) 311 return; 312 if (rcu_is_cpu_rrupt_from_idle()) { 313 rcu_report_exp_rdp(&rcu_sched_state, 314 this_cpu_ptr(&rcu_sched_data), true); 315 return; 316 } 317 __this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true); 318 resched_cpu(smp_processor_id()); 319 } 320 321 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */ 322 static void sync_sched_exp_online_cleanup(int cpu) 323 { 324 struct rcu_data *rdp; 325 int ret; 326 struct rcu_node *rnp; 327 struct rcu_state *rsp = &rcu_sched_state; 328 329 rdp = per_cpu_ptr(rsp->rda, cpu); 330 rnp = rdp->mynode; 331 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask)) 332 return; 333 ret = smp_call_function_single(cpu, sync_sched_exp_handler, rsp, 0); 334 WARN_ON_ONCE(ret); 335 } 336 337 /* 338 * Select the nodes that the upcoming expedited grace period needs 339 * to wait for. 340 */ 341 static void sync_rcu_exp_select_cpus(struct rcu_state *rsp, 342 smp_call_func_t func) 343 { 344 int cpu; 345 unsigned long flags; 346 unsigned long mask_ofl_test; 347 unsigned long mask_ofl_ipi; 348 int ret; 349 struct rcu_node *rnp; 350 351 sync_exp_reset_tree(rsp); 352 rcu_for_each_leaf_node(rsp, rnp) { 353 raw_spin_lock_irqsave_rcu_node(rnp, flags); 354 355 /* Each pass checks a CPU for identity, offline, and idle. */ 356 mask_ofl_test = 0; 357 for_each_leaf_node_possible_cpu(rnp, cpu) { 358 struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu); 359 struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu); 360 361 if (raw_smp_processor_id() == cpu || 362 !(atomic_add_return(0, &rdtp->dynticks) & 0x1) || 363 !(rnp->qsmaskinitnext & rdp->grpmask)) 364 mask_ofl_test |= rdp->grpmask; 365 } 366 mask_ofl_ipi = rnp->expmask & ~mask_ofl_test; 367 368 /* 369 * Need to wait for any blocked tasks as well. Note that 370 * additional blocking tasks will also block the expedited 371 * GP until such time as the ->expmask bits are cleared. 372 */ 373 if (rcu_preempt_has_tasks(rnp)) 374 rnp->exp_tasks = rnp->blkd_tasks.next; 375 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 376 377 /* IPI the remaining CPUs for expedited quiescent state. */ 378 for_each_leaf_node_possible_cpu(rnp, cpu) { 379 unsigned long mask = leaf_node_cpu_bit(rnp, cpu); 380 if (!(mask_ofl_ipi & mask)) 381 continue; 382 retry_ipi: 383 ret = smp_call_function_single(cpu, func, rsp, 0); 384 if (!ret) { 385 mask_ofl_ipi &= ~mask; 386 continue; 387 } 388 /* Failed, raced with CPU hotplug operation. */ 389 raw_spin_lock_irqsave_rcu_node(rnp, flags); 390 if ((rnp->qsmaskinitnext & mask) && 391 (rnp->expmask & mask)) { 392 /* Online, so delay for a bit and try again. */ 393 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 394 schedule_timeout_uninterruptible(1); 395 goto retry_ipi; 396 } 397 /* CPU really is offline, so we can ignore it. */ 398 if (!(rnp->expmask & mask)) 399 mask_ofl_ipi &= ~mask; 400 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 401 } 402 /* Report quiescent states for those that went offline. */ 403 mask_ofl_test |= mask_ofl_ipi; 404 if (mask_ofl_test) 405 rcu_report_exp_cpu_mult(rsp, rnp, mask_ofl_test, false); 406 } 407 } 408 409 static void synchronize_sched_expedited_wait(struct rcu_state *rsp) 410 { 411 int cpu; 412 unsigned long jiffies_stall; 413 unsigned long jiffies_start; 414 unsigned long mask; 415 int ndetected; 416 struct rcu_node *rnp; 417 struct rcu_node *rnp_root = rcu_get_root(rsp); 418 int ret; 419 420 jiffies_stall = rcu_jiffies_till_stall_check(); 421 jiffies_start = jiffies; 422 423 for (;;) { 424 ret = swait_event_timeout( 425 rsp->expedited_wq, 426 sync_rcu_preempt_exp_done(rnp_root), 427 jiffies_stall); 428 if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root)) 429 return; 430 WARN_ON(ret < 0); /* workqueues should not be signaled. */ 431 if (rcu_cpu_stall_suppress) 432 continue; 433 panic_on_rcu_stall(); 434 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {", 435 rsp->name); 436 ndetected = 0; 437 rcu_for_each_leaf_node(rsp, rnp) { 438 ndetected += rcu_print_task_exp_stall(rnp); 439 for_each_leaf_node_possible_cpu(rnp, cpu) { 440 struct rcu_data *rdp; 441 442 mask = leaf_node_cpu_bit(rnp, cpu); 443 if (!(rnp->expmask & mask)) 444 continue; 445 ndetected++; 446 rdp = per_cpu_ptr(rsp->rda, cpu); 447 pr_cont(" %d-%c%c%c", cpu, 448 "O."[!!cpu_online(cpu)], 449 "o."[!!(rdp->grpmask & rnp->expmaskinit)], 450 "N."[!!(rdp->grpmask & rnp->expmaskinitnext)]); 451 } 452 } 453 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n", 454 jiffies - jiffies_start, rsp->expedited_sequence, 455 rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]); 456 if (ndetected) { 457 pr_err("blocking rcu_node structures:"); 458 rcu_for_each_node_breadth_first(rsp, rnp) { 459 if (rnp == rnp_root) 460 continue; /* printed unconditionally */ 461 if (sync_rcu_preempt_exp_done(rnp)) 462 continue; 463 pr_cont(" l=%u:%d-%d:%#lx/%c", 464 rnp->level, rnp->grplo, rnp->grphi, 465 rnp->expmask, 466 ".T"[!!rnp->exp_tasks]); 467 } 468 pr_cont("\n"); 469 } 470 rcu_for_each_leaf_node(rsp, rnp) { 471 for_each_leaf_node_possible_cpu(rnp, cpu) { 472 mask = leaf_node_cpu_bit(rnp, cpu); 473 if (!(rnp->expmask & mask)) 474 continue; 475 dump_cpu_task(cpu); 476 } 477 } 478 jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3; 479 } 480 } 481 482 /* 483 * Wait for the current expedited grace period to complete, and then 484 * wake up everyone who piggybacked on the just-completed expedited 485 * grace period. Also update all the ->exp_seq_rq counters as needed 486 * in order to avoid counter-wrap problems. 487 */ 488 static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s) 489 { 490 struct rcu_node *rnp; 491 492 synchronize_sched_expedited_wait(rsp); 493 rcu_exp_gp_seq_end(rsp); 494 trace_rcu_exp_grace_period(rsp->name, s, TPS("end")); 495 496 /* 497 * Switch over to wakeup mode, allowing the next GP, but -only- the 498 * next GP, to proceed. 499 */ 500 mutex_lock(&rsp->exp_wake_mutex); 501 502 rcu_for_each_node_breadth_first(rsp, rnp) { 503 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) { 504 spin_lock(&rnp->exp_lock); 505 /* Recheck, avoid hang in case someone just arrived. */ 506 if (ULONG_CMP_LT(rnp->exp_seq_rq, s)) 507 rnp->exp_seq_rq = s; 508 spin_unlock(&rnp->exp_lock); 509 } 510 wake_up_all(&rnp->exp_wq[(rsp->expedited_sequence >> 1) & 0x3]); 511 } 512 trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake")); 513 mutex_unlock(&rsp->exp_wake_mutex); 514 } 515 516 /* Let the workqueue handler know what it is supposed to do. */ 517 struct rcu_exp_work { 518 smp_call_func_t rew_func; 519 struct rcu_state *rew_rsp; 520 unsigned long rew_s; 521 struct work_struct rew_work; 522 }; 523 524 /* 525 * Work-queue handler to drive an expedited grace period forward. 526 */ 527 static void wait_rcu_exp_gp(struct work_struct *wp) 528 { 529 struct rcu_exp_work *rewp; 530 531 /* Initialize the rcu_node tree in preparation for the wait. */ 532 rewp = container_of(wp, struct rcu_exp_work, rew_work); 533 sync_rcu_exp_select_cpus(rewp->rew_rsp, rewp->rew_func); 534 535 /* Wait and clean up, including waking everyone. */ 536 rcu_exp_wait_wake(rewp->rew_rsp, rewp->rew_s); 537 } 538 539 /* 540 * Given an rcu_state pointer and a smp_call_function() handler, kick 541 * off the specified flavor of expedited grace period. 542 */ 543 static void _synchronize_rcu_expedited(struct rcu_state *rsp, 544 smp_call_func_t func) 545 { 546 struct rcu_data *rdp; 547 struct rcu_exp_work rew; 548 struct rcu_node *rnp; 549 unsigned long s; 550 551 /* If expedited grace periods are prohibited, fall back to normal. */ 552 if (rcu_gp_is_normal()) { 553 wait_rcu_gp(rsp->call); 554 return; 555 } 556 557 /* Take a snapshot of the sequence number. */ 558 s = rcu_exp_gp_seq_snap(rsp); 559 if (exp_funnel_lock(rsp, s)) 560 return; /* Someone else did our work for us. */ 561 562 /* Marshall arguments and schedule the expedited grace period. */ 563 rew.rew_func = func; 564 rew.rew_rsp = rsp; 565 rew.rew_s = s; 566 INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp); 567 schedule_work(&rew.rew_work); 568 569 /* Wait for expedited grace period to complete. */ 570 rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id()); 571 rnp = rcu_get_root(rsp); 572 wait_event(rnp->exp_wq[(s >> 1) & 0x3], 573 sync_exp_work_done(rsp, 574 &rdp->exp_workdone0, s)); 575 576 /* Let the next expedited grace period start. */ 577 mutex_unlock(&rsp->exp_mutex); 578 } 579 580 /** 581 * synchronize_sched_expedited - Brute-force RCU-sched grace period 582 * 583 * Wait for an RCU-sched grace period to elapse, but use a "big hammer" 584 * approach to force the grace period to end quickly. This consumes 585 * significant time on all CPUs and is unfriendly to real-time workloads, 586 * so is thus not recommended for any sort of common-case code. In fact, 587 * if you are using synchronize_sched_expedited() in a loop, please 588 * restructure your code to batch your updates, and then use a single 589 * synchronize_sched() instead. 590 * 591 * This implementation can be thought of as an application of sequence 592 * locking to expedited grace periods, but using the sequence counter to 593 * determine when someone else has already done the work instead of for 594 * retrying readers. 595 */ 596 void synchronize_sched_expedited(void) 597 { 598 struct rcu_state *rsp = &rcu_sched_state; 599 600 /* If only one CPU, this is automatically a grace period. */ 601 if (rcu_blocking_is_gp()) 602 return; 603 604 _synchronize_rcu_expedited(rsp, sync_sched_exp_handler); 605 } 606 EXPORT_SYMBOL_GPL(synchronize_sched_expedited); 607 608 #ifdef CONFIG_PREEMPT_RCU 609 610 /* 611 * Remote handler for smp_call_function_single(). If there is an 612 * RCU read-side critical section in effect, request that the 613 * next rcu_read_unlock() record the quiescent state up the 614 * ->expmask fields in the rcu_node tree. Otherwise, immediately 615 * report the quiescent state. 616 */ 617 static void sync_rcu_exp_handler(void *info) 618 { 619 struct rcu_data *rdp; 620 struct rcu_state *rsp = info; 621 struct task_struct *t = current; 622 623 /* 624 * Within an RCU read-side critical section, request that the next 625 * rcu_read_unlock() report. Unless this RCU read-side critical 626 * section has already blocked, in which case it is already set 627 * up for the expedited grace period to wait on it. 628 */ 629 if (t->rcu_read_lock_nesting > 0 && 630 !t->rcu_read_unlock_special.b.blocked) { 631 t->rcu_read_unlock_special.b.exp_need_qs = true; 632 return; 633 } 634 635 /* 636 * We are either exiting an RCU read-side critical section (negative 637 * values of t->rcu_read_lock_nesting) or are not in one at all 638 * (zero value of t->rcu_read_lock_nesting). Or we are in an RCU 639 * read-side critical section that blocked before this expedited 640 * grace period started. Either way, we can immediately report 641 * the quiescent state. 642 */ 643 rdp = this_cpu_ptr(rsp->rda); 644 rcu_report_exp_rdp(rsp, rdp, true); 645 } 646 647 /** 648 * synchronize_rcu_expedited - Brute-force RCU grace period 649 * 650 * Wait for an RCU-preempt grace period, but expedite it. The basic 651 * idea is to IPI all non-idle non-nohz online CPUs. The IPI handler 652 * checks whether the CPU is in an RCU-preempt critical section, and 653 * if so, it sets a flag that causes the outermost rcu_read_unlock() 654 * to report the quiescent state. On the other hand, if the CPU is 655 * not in an RCU read-side critical section, the IPI handler reports 656 * the quiescent state immediately. 657 * 658 * Although this is a greate improvement over previous expedited 659 * implementations, it is still unfriendly to real-time workloads, so is 660 * thus not recommended for any sort of common-case code. In fact, if 661 * you are using synchronize_rcu_expedited() in a loop, please restructure 662 * your code to batch your updates, and then Use a single synchronize_rcu() 663 * instead. 664 */ 665 void synchronize_rcu_expedited(void) 666 { 667 struct rcu_state *rsp = rcu_state_p; 668 669 _synchronize_rcu_expedited(rsp, sync_rcu_exp_handler); 670 } 671 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); 672 673 #else /* #ifdef CONFIG_PREEMPT_RCU */ 674 675 /* 676 * Wait for an rcu-preempt grace period, but make it happen quickly. 677 * But because preemptible RCU does not exist, map to rcu-sched. 678 */ 679 void synchronize_rcu_expedited(void) 680 { 681 synchronize_sched_expedited(); 682 } 683 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); 684 685 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */ 686