1 /* SPDX-License-Identifier: GPL-2.0+ */ 2 /* 3 * RCU expedited grace periods 4 * 5 * Copyright IBM Corporation, 2016 6 * 7 * Authors: Paul E. McKenney <paulmck@linux.ibm.com> 8 */ 9 10 #include <linux/lockdep.h> 11 12 static void rcu_exp_handler(void *unused); 13 static int rcu_print_task_exp_stall(struct rcu_node *rnp); 14 15 /* 16 * Record the start of an expedited grace period. 17 */ 18 static void rcu_exp_gp_seq_start(void) 19 { 20 rcu_seq_start(&rcu_state.expedited_sequence); 21 } 22 23 /* 24 * Return the value that the expedited-grace-period counter will have 25 * at the end of the current grace period. 26 */ 27 static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void) 28 { 29 return rcu_seq_endval(&rcu_state.expedited_sequence); 30 } 31 32 /* 33 * Record the end of an expedited grace period. 34 */ 35 static void rcu_exp_gp_seq_end(void) 36 { 37 rcu_seq_end(&rcu_state.expedited_sequence); 38 smp_mb(); /* Ensure that consecutive grace periods serialize. */ 39 } 40 41 /* 42 * Take a snapshot of the expedited-grace-period counter, which is the 43 * earliest value that will indicate that a full grace period has 44 * elapsed since the current time. 45 */ 46 static unsigned long rcu_exp_gp_seq_snap(void) 47 { 48 unsigned long s; 49 50 smp_mb(); /* Caller's modifications seen first by other CPUs. */ 51 s = rcu_seq_snap(&rcu_state.expedited_sequence); 52 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap")); 53 return s; 54 } 55 56 /* 57 * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true 58 * if a full expedited grace period has elapsed since that snapshot 59 * was taken. 60 */ 61 static bool rcu_exp_gp_seq_done(unsigned long s) 62 { 63 return rcu_seq_done(&rcu_state.expedited_sequence, s); 64 } 65 66 /* 67 * Reset the ->expmaskinit values in the rcu_node tree to reflect any 68 * recent CPU-online activity. Note that these masks are not cleared 69 * when CPUs go offline, so they reflect the union of all CPUs that have 70 * ever been online. This means that this function normally takes its 71 * no-work-to-do fastpath. 72 */ 73 static void sync_exp_reset_tree_hotplug(void) 74 { 75 bool done; 76 unsigned long flags; 77 unsigned long mask; 78 unsigned long oldmask; 79 int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */ 80 struct rcu_node *rnp; 81 struct rcu_node *rnp_up; 82 83 /* If no new CPUs onlined since last time, nothing to do. */ 84 if (likely(ncpus == rcu_state.ncpus_snap)) 85 return; 86 rcu_state.ncpus_snap = ncpus; 87 88 /* 89 * Each pass through the following loop propagates newly onlined 90 * CPUs for the current rcu_node structure up the rcu_node tree. 91 */ 92 rcu_for_each_leaf_node(rnp) { 93 raw_spin_lock_irqsave_rcu_node(rnp, flags); 94 if (rnp->expmaskinit == rnp->expmaskinitnext) { 95 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 96 continue; /* No new CPUs, nothing to do. */ 97 } 98 99 /* Update this node's mask, track old value for propagation. */ 100 oldmask = rnp->expmaskinit; 101 rnp->expmaskinit = rnp->expmaskinitnext; 102 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 103 104 /* If was already nonzero, nothing to propagate. */ 105 if (oldmask) 106 continue; 107 108 /* Propagate the new CPU up the tree. */ 109 mask = rnp->grpmask; 110 rnp_up = rnp->parent; 111 done = false; 112 while (rnp_up) { 113 raw_spin_lock_irqsave_rcu_node(rnp_up, flags); 114 if (rnp_up->expmaskinit) 115 done = true; 116 rnp_up->expmaskinit |= mask; 117 raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags); 118 if (done) 119 break; 120 mask = rnp_up->grpmask; 121 rnp_up = rnp_up->parent; 122 } 123 } 124 } 125 126 /* 127 * Reset the ->expmask values in the rcu_node tree in preparation for 128 * a new expedited grace period. 129 */ 130 static void __maybe_unused sync_exp_reset_tree(void) 131 { 132 unsigned long flags; 133 struct rcu_node *rnp; 134 135 sync_exp_reset_tree_hotplug(); 136 rcu_for_each_node_breadth_first(rnp) { 137 raw_spin_lock_irqsave_rcu_node(rnp, flags); 138 WARN_ON_ONCE(rnp->expmask); 139 WRITE_ONCE(rnp->expmask, rnp->expmaskinit); 140 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 141 } 142 } 143 144 /* 145 * Return non-zero if there is no RCU expedited grace period in progress 146 * for the specified rcu_node structure, in other words, if all CPUs and 147 * tasks covered by the specified rcu_node structure have done their bit 148 * for the current expedited grace period. 149 */ 150 static bool sync_rcu_exp_done(struct rcu_node *rnp) 151 { 152 raw_lockdep_assert_held_rcu_node(rnp); 153 return READ_ONCE(rnp->exp_tasks) == NULL && 154 READ_ONCE(rnp->expmask) == 0; 155 } 156 157 /* 158 * Like sync_rcu_exp_done(), but where the caller does not hold the 159 * rcu_node's ->lock. 160 */ 161 static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp) 162 { 163 unsigned long flags; 164 bool ret; 165 166 raw_spin_lock_irqsave_rcu_node(rnp, flags); 167 ret = sync_rcu_exp_done(rnp); 168 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 169 170 return ret; 171 } 172 173 174 /* 175 * Report the exit from RCU read-side critical section for the last task 176 * that queued itself during or before the current expedited preemptible-RCU 177 * grace period. This event is reported either to the rcu_node structure on 178 * which the task was queued or to one of that rcu_node structure's ancestors, 179 * recursively up the tree. (Calm down, calm down, we do the recursion 180 * iteratively!) 181 */ 182 static void __rcu_report_exp_rnp(struct rcu_node *rnp, 183 bool wake, unsigned long flags) 184 __releases(rnp->lock) 185 { 186 unsigned long mask; 187 188 raw_lockdep_assert_held_rcu_node(rnp); 189 for (;;) { 190 if (!sync_rcu_exp_done(rnp)) { 191 if (!rnp->expmask) 192 rcu_initiate_boost(rnp, flags); 193 else 194 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 195 break; 196 } 197 if (rnp->parent == NULL) { 198 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 199 if (wake) { 200 smp_mb(); /* EGP done before wake_up(). */ 201 swake_up_one(&rcu_state.expedited_wq); 202 } 203 break; 204 } 205 mask = rnp->grpmask; 206 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */ 207 rnp = rnp->parent; 208 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */ 209 WARN_ON_ONCE(!(rnp->expmask & mask)); 210 WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask); 211 } 212 } 213 214 /* 215 * Report expedited quiescent state for specified node. This is a 216 * lock-acquisition wrapper function for __rcu_report_exp_rnp(). 217 */ 218 static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake) 219 { 220 unsigned long flags; 221 222 raw_spin_lock_irqsave_rcu_node(rnp, flags); 223 __rcu_report_exp_rnp(rnp, wake, flags); 224 } 225 226 /* 227 * Report expedited quiescent state for multiple CPUs, all covered by the 228 * specified leaf rcu_node structure. 229 */ 230 static void rcu_report_exp_cpu_mult(struct rcu_node *rnp, 231 unsigned long mask, bool wake) 232 { 233 int cpu; 234 unsigned long flags; 235 struct rcu_data *rdp; 236 237 raw_spin_lock_irqsave_rcu_node(rnp, flags); 238 if (!(rnp->expmask & mask)) { 239 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 240 return; 241 } 242 WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask); 243 for_each_leaf_node_cpu_mask(rnp, cpu, mask) { 244 rdp = per_cpu_ptr(&rcu_data, cpu); 245 if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp) 246 continue; 247 rdp->rcu_forced_tick_exp = false; 248 tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP); 249 } 250 __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */ 251 } 252 253 /* 254 * Report expedited quiescent state for specified rcu_data (CPU). 255 */ 256 static void rcu_report_exp_rdp(struct rcu_data *rdp) 257 { 258 WRITE_ONCE(rdp->exp_deferred_qs, false); 259 rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true); 260 } 261 262 /* Common code for work-done checking. */ 263 static bool sync_exp_work_done(unsigned long s) 264 { 265 if (rcu_exp_gp_seq_done(s)) { 266 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done")); 267 smp_mb(); /* Ensure test happens before caller kfree(). */ 268 return true; 269 } 270 return false; 271 } 272 273 /* 274 * Funnel-lock acquisition for expedited grace periods. Returns true 275 * if some other task completed an expedited grace period that this task 276 * can piggy-back on, and with no mutex held. Otherwise, returns false 277 * with the mutex held, indicating that the caller must actually do the 278 * expedited grace period. 279 */ 280 static bool exp_funnel_lock(unsigned long s) 281 { 282 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id()); 283 struct rcu_node *rnp = rdp->mynode; 284 struct rcu_node *rnp_root = rcu_get_root(); 285 286 /* Low-contention fastpath. */ 287 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) && 288 (rnp == rnp_root || 289 ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) && 290 mutex_trylock(&rcu_state.exp_mutex)) 291 goto fastpath; 292 293 /* 294 * Each pass through the following loop works its way up 295 * the rcu_node tree, returning if others have done the work or 296 * otherwise falls through to acquire ->exp_mutex. The mapping 297 * from CPU to rcu_node structure can be inexact, as it is just 298 * promoting locality and is not strictly needed for correctness. 299 */ 300 for (; rnp != NULL; rnp = rnp->parent) { 301 if (sync_exp_work_done(s)) 302 return true; 303 304 /* Work not done, either wait here or go up. */ 305 spin_lock(&rnp->exp_lock); 306 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) { 307 308 /* Someone else doing GP, so wait for them. */ 309 spin_unlock(&rnp->exp_lock); 310 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level, 311 rnp->grplo, rnp->grphi, 312 TPS("wait")); 313 wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3], 314 sync_exp_work_done(s)); 315 return true; 316 } 317 WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */ 318 spin_unlock(&rnp->exp_lock); 319 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level, 320 rnp->grplo, rnp->grphi, TPS("nxtlvl")); 321 } 322 mutex_lock(&rcu_state.exp_mutex); 323 fastpath: 324 if (sync_exp_work_done(s)) { 325 mutex_unlock(&rcu_state.exp_mutex); 326 return true; 327 } 328 rcu_exp_gp_seq_start(); 329 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start")); 330 return false; 331 } 332 333 /* 334 * Select the CPUs within the specified rcu_node that the upcoming 335 * expedited grace period needs to wait for. 336 */ 337 static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) 338 { 339 int cpu; 340 unsigned long flags; 341 unsigned long mask_ofl_test; 342 unsigned long mask_ofl_ipi; 343 int ret; 344 struct rcu_exp_work *rewp = 345 container_of(wp, struct rcu_exp_work, rew_work); 346 struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew); 347 348 raw_spin_lock_irqsave_rcu_node(rnp, flags); 349 350 /* Each pass checks a CPU for identity, offline, and idle. */ 351 mask_ofl_test = 0; 352 for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) { 353 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); 354 unsigned long mask = rdp->grpmask; 355 int snap; 356 357 if (raw_smp_processor_id() == cpu || 358 !(rnp->qsmaskinitnext & mask)) { 359 mask_ofl_test |= mask; 360 } else { 361 snap = rcu_dynticks_snap(rdp); 362 if (rcu_dynticks_in_eqs(snap)) 363 mask_ofl_test |= mask; 364 else 365 rdp->exp_dynticks_snap = snap; 366 } 367 } 368 mask_ofl_ipi = rnp->expmask & ~mask_ofl_test; 369 370 /* 371 * Need to wait for any blocked tasks as well. Note that 372 * additional blocking tasks will also block the expedited GP 373 * until such time as the ->expmask bits are cleared. 374 */ 375 if (rcu_preempt_has_tasks(rnp)) 376 WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next); 377 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 378 379 /* IPI the remaining CPUs for expedited quiescent state. */ 380 for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) { 381 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); 382 unsigned long mask = rdp->grpmask; 383 384 retry_ipi: 385 if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) { 386 mask_ofl_test |= mask; 387 continue; 388 } 389 if (get_cpu() == cpu) { 390 put_cpu(); 391 continue; 392 } 393 ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0); 394 put_cpu(); 395 /* The CPU will report the QS in response to the IPI. */ 396 if (!ret) 397 continue; 398 399 /* Failed, raced with CPU hotplug operation. */ 400 raw_spin_lock_irqsave_rcu_node(rnp, flags); 401 if ((rnp->qsmaskinitnext & mask) && 402 (rnp->expmask & mask)) { 403 /* Online, so delay for a bit and try again. */ 404 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 405 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl")); 406 schedule_timeout_idle(1); 407 goto retry_ipi; 408 } 409 /* CPU really is offline, so we must report its QS. */ 410 if (rnp->expmask & mask) 411 mask_ofl_test |= mask; 412 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 413 } 414 /* Report quiescent states for those that went offline. */ 415 if (mask_ofl_test) 416 rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false); 417 } 418 419 /* 420 * Select the nodes that the upcoming expedited grace period needs 421 * to wait for. 422 */ 423 static void sync_rcu_exp_select_cpus(void) 424 { 425 int cpu; 426 struct rcu_node *rnp; 427 428 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset")); 429 sync_exp_reset_tree(); 430 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select")); 431 432 /* Schedule work for each leaf rcu_node structure. */ 433 rcu_for_each_leaf_node(rnp) { 434 rnp->exp_need_flush = false; 435 if (!READ_ONCE(rnp->expmask)) 436 continue; /* Avoid early boot non-existent wq. */ 437 if (!READ_ONCE(rcu_par_gp_wq) || 438 rcu_scheduler_active != RCU_SCHEDULER_RUNNING || 439 rcu_is_last_leaf_node(rnp)) { 440 /* No workqueues yet or last leaf, do direct call. */ 441 sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work); 442 continue; 443 } 444 INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus); 445 cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1); 446 /* If all offline, queue the work on an unbound CPU. */ 447 if (unlikely(cpu > rnp->grphi - rnp->grplo)) 448 cpu = WORK_CPU_UNBOUND; 449 else 450 cpu += rnp->grplo; 451 queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work); 452 rnp->exp_need_flush = true; 453 } 454 455 /* Wait for workqueue jobs (if any) to complete. */ 456 rcu_for_each_leaf_node(rnp) 457 if (rnp->exp_need_flush) 458 flush_work(&rnp->rew.rew_work); 459 } 460 461 /* 462 * Wait for the expedited grace period to elapse, within time limit. 463 * If the time limit is exceeded without the grace period elapsing, 464 * return false, otherwise return true. 465 */ 466 static bool synchronize_rcu_expedited_wait_once(long tlimit) 467 { 468 int t; 469 struct rcu_node *rnp_root = rcu_get_root(); 470 471 t = swait_event_timeout_exclusive(rcu_state.expedited_wq, 472 sync_rcu_exp_done_unlocked(rnp_root), 473 tlimit); 474 // Workqueues should not be signaled. 475 if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root)) 476 return true; 477 WARN_ON(t < 0); /* workqueues should not be signaled. */ 478 return false; 479 } 480 481 /* 482 * Wait for the expedited grace period to elapse, issuing any needed 483 * RCU CPU stall warnings along the way. 484 */ 485 static void synchronize_rcu_expedited_wait(void) 486 { 487 int cpu; 488 unsigned long j; 489 unsigned long jiffies_stall; 490 unsigned long jiffies_start; 491 unsigned long mask; 492 int ndetected; 493 struct rcu_data *rdp; 494 struct rcu_node *rnp; 495 struct rcu_node *rnp_root = rcu_get_root(); 496 497 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait")); 498 jiffies_stall = rcu_jiffies_till_stall_check(); 499 jiffies_start = jiffies; 500 if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) { 501 if (synchronize_rcu_expedited_wait_once(1)) 502 return; 503 rcu_for_each_leaf_node(rnp) { 504 for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) { 505 rdp = per_cpu_ptr(&rcu_data, cpu); 506 if (rdp->rcu_forced_tick_exp) 507 continue; 508 rdp->rcu_forced_tick_exp = true; 509 tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP); 510 } 511 } 512 j = READ_ONCE(jiffies_till_first_fqs); 513 if (synchronize_rcu_expedited_wait_once(j + HZ)) 514 return; 515 WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT)); 516 } 517 518 for (;;) { 519 if (synchronize_rcu_expedited_wait_once(jiffies_stall)) 520 return; 521 if (rcu_stall_is_suppressed()) 522 continue; 523 panic_on_rcu_stall(); 524 trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall")); 525 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {", 526 rcu_state.name); 527 ndetected = 0; 528 rcu_for_each_leaf_node(rnp) { 529 ndetected += rcu_print_task_exp_stall(rnp); 530 for_each_leaf_node_possible_cpu(rnp, cpu) { 531 struct rcu_data *rdp; 532 533 mask = leaf_node_cpu_bit(rnp, cpu); 534 if (!(READ_ONCE(rnp->expmask) & mask)) 535 continue; 536 ndetected++; 537 rdp = per_cpu_ptr(&rcu_data, cpu); 538 pr_cont(" %d-%c%c%c", cpu, 539 "O."[!!cpu_online(cpu)], 540 "o."[!!(rdp->grpmask & rnp->expmaskinit)], 541 "N."[!!(rdp->grpmask & rnp->expmaskinitnext)]); 542 } 543 } 544 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n", 545 jiffies - jiffies_start, rcu_state.expedited_sequence, 546 data_race(rnp_root->expmask), 547 ".T"[!!data_race(rnp_root->exp_tasks)]); 548 if (ndetected) { 549 pr_err("blocking rcu_node structures (internal RCU debug):"); 550 rcu_for_each_node_breadth_first(rnp) { 551 if (rnp == rnp_root) 552 continue; /* printed unconditionally */ 553 if (sync_rcu_exp_done_unlocked(rnp)) 554 continue; 555 pr_cont(" l=%u:%d-%d:%#lx/%c", 556 rnp->level, rnp->grplo, rnp->grphi, 557 data_race(rnp->expmask), 558 ".T"[!!data_race(rnp->exp_tasks)]); 559 } 560 pr_cont("\n"); 561 } 562 rcu_for_each_leaf_node(rnp) { 563 for_each_leaf_node_possible_cpu(rnp, cpu) { 564 mask = leaf_node_cpu_bit(rnp, cpu); 565 if (!(READ_ONCE(rnp->expmask) & mask)) 566 continue; 567 dump_cpu_task(cpu); 568 } 569 } 570 jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3; 571 } 572 } 573 574 /* 575 * Wait for the current expedited grace period to complete, and then 576 * wake up everyone who piggybacked on the just-completed expedited 577 * grace period. Also update all the ->exp_seq_rq counters as needed 578 * in order to avoid counter-wrap problems. 579 */ 580 static void rcu_exp_wait_wake(unsigned long s) 581 { 582 struct rcu_node *rnp; 583 584 synchronize_rcu_expedited_wait(); 585 586 // Switch over to wakeup mode, allowing the next GP to proceed. 587 // End the previous grace period only after acquiring the mutex 588 // to ensure that only one GP runs concurrently with wakeups. 589 mutex_lock(&rcu_state.exp_wake_mutex); 590 rcu_exp_gp_seq_end(); 591 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end")); 592 593 rcu_for_each_node_breadth_first(rnp) { 594 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) { 595 spin_lock(&rnp->exp_lock); 596 /* Recheck, avoid hang in case someone just arrived. */ 597 if (ULONG_CMP_LT(rnp->exp_seq_rq, s)) 598 WRITE_ONCE(rnp->exp_seq_rq, s); 599 spin_unlock(&rnp->exp_lock); 600 } 601 smp_mb(); /* All above changes before wakeup. */ 602 wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]); 603 } 604 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake")); 605 mutex_unlock(&rcu_state.exp_wake_mutex); 606 } 607 608 /* 609 * Common code to drive an expedited grace period forward, used by 610 * workqueues and mid-boot-time tasks. 611 */ 612 static void rcu_exp_sel_wait_wake(unsigned long s) 613 { 614 /* Initialize the rcu_node tree in preparation for the wait. */ 615 sync_rcu_exp_select_cpus(); 616 617 /* Wait and clean up, including waking everyone. */ 618 rcu_exp_wait_wake(s); 619 } 620 621 /* 622 * Work-queue handler to drive an expedited grace period forward. 623 */ 624 static void wait_rcu_exp_gp(struct work_struct *wp) 625 { 626 struct rcu_exp_work *rewp; 627 628 rewp = container_of(wp, struct rcu_exp_work, rew_work); 629 rcu_exp_sel_wait_wake(rewp->rew_s); 630 } 631 632 #ifdef CONFIG_PREEMPT_RCU 633 634 /* 635 * Remote handler for smp_call_function_single(). If there is an 636 * RCU read-side critical section in effect, request that the 637 * next rcu_read_unlock() record the quiescent state up the 638 * ->expmask fields in the rcu_node tree. Otherwise, immediately 639 * report the quiescent state. 640 */ 641 static void rcu_exp_handler(void *unused) 642 { 643 int depth = rcu_preempt_depth(); 644 unsigned long flags; 645 struct rcu_data *rdp = this_cpu_ptr(&rcu_data); 646 struct rcu_node *rnp = rdp->mynode; 647 struct task_struct *t = current; 648 649 /* 650 * First, the common case of not being in an RCU read-side 651 * critical section. If also enabled or idle, immediately 652 * report the quiescent state, otherwise defer. 653 */ 654 if (!depth) { 655 if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || 656 rcu_dynticks_curr_cpu_in_eqs()) { 657 rcu_report_exp_rdp(rdp); 658 } else { 659 rdp->exp_deferred_qs = true; 660 set_tsk_need_resched(t); 661 set_preempt_need_resched(); 662 } 663 return; 664 } 665 666 /* 667 * Second, the less-common case of being in an RCU read-side 668 * critical section. In this case we can count on a future 669 * rcu_read_unlock(). However, this rcu_read_unlock() might 670 * execute on some other CPU, but in that case there will be 671 * a future context switch. Either way, if the expedited 672 * grace period is still waiting on this CPU, set ->deferred_qs 673 * so that the eventual quiescent state will be reported. 674 * Note that there is a large group of race conditions that 675 * can have caused this quiescent state to already have been 676 * reported, so we really do need to check ->expmask. 677 */ 678 if (depth > 0) { 679 raw_spin_lock_irqsave_rcu_node(rnp, flags); 680 if (rnp->expmask & rdp->grpmask) { 681 rdp->exp_deferred_qs = true; 682 t->rcu_read_unlock_special.b.exp_hint = true; 683 } 684 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 685 return; 686 } 687 688 // Finally, negative nesting depth should not happen. 689 WARN_ON_ONCE(1); 690 } 691 692 /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */ 693 static void sync_sched_exp_online_cleanup(int cpu) 694 { 695 } 696 697 /* 698 * Scan the current list of tasks blocked within RCU read-side critical 699 * sections, printing out the tid of each that is blocking the current 700 * expedited grace period. 701 */ 702 static int rcu_print_task_exp_stall(struct rcu_node *rnp) 703 { 704 unsigned long flags; 705 int ndetected = 0; 706 struct task_struct *t; 707 708 if (!READ_ONCE(rnp->exp_tasks)) 709 return 0; 710 raw_spin_lock_irqsave_rcu_node(rnp, flags); 711 t = list_entry(rnp->exp_tasks->prev, 712 struct task_struct, rcu_node_entry); 713 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) { 714 pr_cont(" P%d", t->pid); 715 ndetected++; 716 } 717 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 718 return ndetected; 719 } 720 721 #else /* #ifdef CONFIG_PREEMPT_RCU */ 722 723 /* Request an expedited quiescent state. */ 724 static void rcu_exp_need_qs(void) 725 { 726 __this_cpu_write(rcu_data.cpu_no_qs.b.exp, true); 727 /* Store .exp before .rcu_urgent_qs. */ 728 smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true); 729 set_tsk_need_resched(current); 730 set_preempt_need_resched(); 731 } 732 733 /* Invoked on each online non-idle CPU for expedited quiescent state. */ 734 static void rcu_exp_handler(void *unused) 735 { 736 struct rcu_data *rdp = this_cpu_ptr(&rcu_data); 737 struct rcu_node *rnp = rdp->mynode; 738 739 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) || 740 __this_cpu_read(rcu_data.cpu_no_qs.b.exp)) 741 return; 742 if (rcu_is_cpu_rrupt_from_idle()) { 743 rcu_report_exp_rdp(this_cpu_ptr(&rcu_data)); 744 return; 745 } 746 rcu_exp_need_qs(); 747 } 748 749 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */ 750 static void sync_sched_exp_online_cleanup(int cpu) 751 { 752 unsigned long flags; 753 int my_cpu; 754 struct rcu_data *rdp; 755 int ret; 756 struct rcu_node *rnp; 757 758 rdp = per_cpu_ptr(&rcu_data, cpu); 759 rnp = rdp->mynode; 760 my_cpu = get_cpu(); 761 /* Quiescent state either not needed or already requested, leave. */ 762 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) || 763 __this_cpu_read(rcu_data.cpu_no_qs.b.exp)) { 764 put_cpu(); 765 return; 766 } 767 /* Quiescent state needed on current CPU, so set it up locally. */ 768 if (my_cpu == cpu) { 769 local_irq_save(flags); 770 rcu_exp_need_qs(); 771 local_irq_restore(flags); 772 put_cpu(); 773 return; 774 } 775 /* Quiescent state needed on some other CPU, send IPI. */ 776 ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0); 777 put_cpu(); 778 WARN_ON_ONCE(ret); 779 } 780 781 /* 782 * Because preemptible RCU does not exist, we never have to check for 783 * tasks blocked within RCU read-side critical sections that are 784 * blocking the current expedited grace period. 785 */ 786 static int rcu_print_task_exp_stall(struct rcu_node *rnp) 787 { 788 return 0; 789 } 790 791 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */ 792 793 /** 794 * synchronize_rcu_expedited - Brute-force RCU grace period 795 * 796 * Wait for an RCU grace period, but expedite it. The basic idea is to 797 * IPI all non-idle non-nohz online CPUs. The IPI handler checks whether 798 * the CPU is in an RCU critical section, and if so, it sets a flag that 799 * causes the outermost rcu_read_unlock() to report the quiescent state 800 * for RCU-preempt or asks the scheduler for help for RCU-sched. On the 801 * other hand, if the CPU is not in an RCU read-side critical section, 802 * the IPI handler reports the quiescent state immediately. 803 * 804 * Although this is a great improvement over previous expedited 805 * implementations, it is still unfriendly to real-time workloads, so is 806 * thus not recommended for any sort of common-case code. In fact, if 807 * you are using synchronize_rcu_expedited() in a loop, please restructure 808 * your code to batch your updates, and then use a single synchronize_rcu() 809 * instead. 810 * 811 * This has the same semantics as (but is more brutal than) synchronize_rcu(). 812 */ 813 void synchronize_rcu_expedited(void) 814 { 815 bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT); 816 struct rcu_exp_work rew; 817 struct rcu_node *rnp; 818 unsigned long s; 819 820 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) || 821 lock_is_held(&rcu_lock_map) || 822 lock_is_held(&rcu_sched_lock_map), 823 "Illegal synchronize_rcu_expedited() in RCU read-side critical section"); 824 825 /* Is the state is such that the call is a grace period? */ 826 if (rcu_blocking_is_gp()) 827 return; 828 829 /* If expedited grace periods are prohibited, fall back to normal. */ 830 if (rcu_gp_is_normal()) { 831 wait_rcu_gp(call_rcu); 832 return; 833 } 834 835 /* Take a snapshot of the sequence number. */ 836 s = rcu_exp_gp_seq_snap(); 837 if (exp_funnel_lock(s)) 838 return; /* Someone else did our work for us. */ 839 840 /* Ensure that load happens before action based on it. */ 841 if (unlikely(boottime)) { 842 /* Direct call during scheduler init and early_initcalls(). */ 843 rcu_exp_sel_wait_wake(s); 844 } else { 845 /* Marshall arguments & schedule the expedited grace period. */ 846 rew.rew_s = s; 847 INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp); 848 queue_work(rcu_gp_wq, &rew.rew_work); 849 } 850 851 /* Wait for expedited grace period to complete. */ 852 rnp = rcu_get_root(); 853 wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3], 854 sync_exp_work_done(s)); 855 smp_mb(); /* Workqueue actions happen before return. */ 856 857 /* Let the next expedited grace period start. */ 858 mutex_unlock(&rcu_state.exp_mutex); 859 860 if (likely(!boottime)) 861 destroy_work_on_stack(&rew.rew_work); 862 } 863 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); 864