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.h> 40 #include <linux/atomic.h> 41 #include <linux/bitops.h> 42 #include <linux/percpu.h> 43 #include <linux/notifier.h> 44 #include <linux/cpu.h> 45 #include <linux/mutex.h> 46 #include <linux/export.h> 47 #include <linux/hardirq.h> 48 #include <linux/delay.h> 49 #include <linux/module.h> 50 #include <linux/kthread.h> 51 #include <linux/tick.h> 52 53 #define CREATE_TRACE_POINTS 54 55 #include "rcu.h" 56 57 MODULE_ALIAS("rcupdate"); 58 #ifdef MODULE_PARAM_PREFIX 59 #undef MODULE_PARAM_PREFIX 60 #endif 61 #define MODULE_PARAM_PREFIX "rcupdate." 62 63 module_param(rcu_expedited, int, 0); 64 65 #ifdef CONFIG_PREEMPT_RCU 66 67 /* 68 * Preemptible RCU implementation for rcu_read_lock(). 69 * Just increment ->rcu_read_lock_nesting, shared state will be updated 70 * if we block. 71 */ 72 void __rcu_read_lock(void) 73 { 74 current->rcu_read_lock_nesting++; 75 barrier(); /* critical section after entry code. */ 76 } 77 EXPORT_SYMBOL_GPL(__rcu_read_lock); 78 79 /* 80 * Preemptible RCU implementation for rcu_read_unlock(). 81 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost 82 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then 83 * invoke rcu_read_unlock_special() to clean up after a context switch 84 * in an RCU read-side critical section and other special cases. 85 */ 86 void __rcu_read_unlock(void) 87 { 88 struct task_struct *t = current; 89 90 if (t->rcu_read_lock_nesting != 1) { 91 --t->rcu_read_lock_nesting; 92 } else { 93 barrier(); /* critical section before exit code. */ 94 t->rcu_read_lock_nesting = INT_MIN; 95 barrier(); /* assign before ->rcu_read_unlock_special load */ 96 if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special.s))) 97 rcu_read_unlock_special(t); 98 barrier(); /* ->rcu_read_unlock_special load before assign */ 99 t->rcu_read_lock_nesting = 0; 100 } 101 #ifdef CONFIG_PROVE_LOCKING 102 { 103 int rrln = ACCESS_ONCE(t->rcu_read_lock_nesting); 104 105 WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2); 106 } 107 #endif /* #ifdef CONFIG_PROVE_LOCKING */ 108 } 109 EXPORT_SYMBOL_GPL(__rcu_read_unlock); 110 111 #endif /* #ifdef CONFIG_PREEMPT_RCU */ 112 113 #ifdef CONFIG_DEBUG_LOCK_ALLOC 114 static struct lock_class_key rcu_lock_key; 115 struct lockdep_map rcu_lock_map = 116 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key); 117 EXPORT_SYMBOL_GPL(rcu_lock_map); 118 119 static struct lock_class_key rcu_bh_lock_key; 120 struct lockdep_map rcu_bh_lock_map = 121 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key); 122 EXPORT_SYMBOL_GPL(rcu_bh_lock_map); 123 124 static struct lock_class_key rcu_sched_lock_key; 125 struct lockdep_map rcu_sched_lock_map = 126 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key); 127 EXPORT_SYMBOL_GPL(rcu_sched_lock_map); 128 129 static struct lock_class_key rcu_callback_key; 130 struct lockdep_map rcu_callback_map = 131 STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key); 132 EXPORT_SYMBOL_GPL(rcu_callback_map); 133 134 int notrace debug_lockdep_rcu_enabled(void) 135 { 136 return rcu_scheduler_active && debug_locks && 137 current->lockdep_recursion == 0; 138 } 139 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled); 140 141 /** 142 * rcu_read_lock_held() - might we be in RCU read-side critical section? 143 * 144 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU 145 * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC, 146 * this assumes we are in an RCU read-side critical section unless it can 147 * prove otherwise. This is useful for debug checks in functions that 148 * require that they be called within an RCU read-side critical section. 149 * 150 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot 151 * and while lockdep is disabled. 152 * 153 * Note that rcu_read_lock() and the matching rcu_read_unlock() must 154 * occur in the same context, for example, it is illegal to invoke 155 * rcu_read_unlock() in process context if the matching rcu_read_lock() 156 * was invoked from within an irq handler. 157 * 158 * Note that rcu_read_lock() is disallowed if the CPU is either idle or 159 * offline from an RCU perspective, so check for those as well. 160 */ 161 int rcu_read_lock_held(void) 162 { 163 if (!debug_lockdep_rcu_enabled()) 164 return 1; 165 if (!rcu_is_watching()) 166 return 0; 167 if (!rcu_lockdep_current_cpu_online()) 168 return 0; 169 return lock_is_held(&rcu_lock_map); 170 } 171 EXPORT_SYMBOL_GPL(rcu_read_lock_held); 172 173 /** 174 * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section? 175 * 176 * Check for bottom half being disabled, which covers both the 177 * CONFIG_PROVE_RCU and not cases. Note that if someone uses 178 * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled) 179 * will show the situation. This is useful for debug checks in functions 180 * that require that they be called within an RCU read-side critical 181 * section. 182 * 183 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot. 184 * 185 * Note that rcu_read_lock() is disallowed if the CPU is either idle or 186 * offline from an RCU perspective, so check for those as well. 187 */ 188 int rcu_read_lock_bh_held(void) 189 { 190 if (!debug_lockdep_rcu_enabled()) 191 return 1; 192 if (!rcu_is_watching()) 193 return 0; 194 if (!rcu_lockdep_current_cpu_online()) 195 return 0; 196 return in_softirq() || irqs_disabled(); 197 } 198 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held); 199 200 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ 201 202 struct rcu_synchronize { 203 struct rcu_head head; 204 struct completion completion; 205 }; 206 207 /* 208 * Awaken the corresponding synchronize_rcu() instance now that a 209 * grace period has elapsed. 210 */ 211 static void wakeme_after_rcu(struct rcu_head *head) 212 { 213 struct rcu_synchronize *rcu; 214 215 rcu = container_of(head, struct rcu_synchronize, head); 216 complete(&rcu->completion); 217 } 218 219 void wait_rcu_gp(call_rcu_func_t crf) 220 { 221 struct rcu_synchronize rcu; 222 223 init_rcu_head_on_stack(&rcu.head); 224 init_completion(&rcu.completion); 225 /* Will wake me after RCU finished. */ 226 crf(&rcu.head, wakeme_after_rcu); 227 /* Wait for it. */ 228 wait_for_completion(&rcu.completion); 229 destroy_rcu_head_on_stack(&rcu.head); 230 } 231 EXPORT_SYMBOL_GPL(wait_rcu_gp); 232 233 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD 234 void init_rcu_head(struct rcu_head *head) 235 { 236 debug_object_init(head, &rcuhead_debug_descr); 237 } 238 239 void destroy_rcu_head(struct rcu_head *head) 240 { 241 debug_object_free(head, &rcuhead_debug_descr); 242 } 243 244 /* 245 * fixup_activate is called when: 246 * - an active object is activated 247 * - an unknown object is activated (might be a statically initialized object) 248 * Activation is performed internally by call_rcu(). 249 */ 250 static int rcuhead_fixup_activate(void *addr, enum debug_obj_state state) 251 { 252 struct rcu_head *head = addr; 253 254 switch (state) { 255 256 case ODEBUG_STATE_NOTAVAILABLE: 257 /* 258 * This is not really a fixup. We just make sure that it is 259 * tracked in the object tracker. 260 */ 261 debug_object_init(head, &rcuhead_debug_descr); 262 debug_object_activate(head, &rcuhead_debug_descr); 263 return 0; 264 default: 265 return 1; 266 } 267 } 268 269 /** 270 * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects 271 * @head: pointer to rcu_head structure to be initialized 272 * 273 * This function informs debugobjects of a new rcu_head structure that 274 * has been allocated as an auto variable on the stack. This function 275 * is not required for rcu_head structures that are statically defined or 276 * that are dynamically allocated on the heap. This function has no 277 * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. 278 */ 279 void init_rcu_head_on_stack(struct rcu_head *head) 280 { 281 debug_object_init_on_stack(head, &rcuhead_debug_descr); 282 } 283 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack); 284 285 /** 286 * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects 287 * @head: pointer to rcu_head structure to be initialized 288 * 289 * This function informs debugobjects that an on-stack rcu_head structure 290 * is about to go out of scope. As with init_rcu_head_on_stack(), this 291 * function is not required for rcu_head structures that are statically 292 * defined or that are dynamically allocated on the heap. Also as with 293 * init_rcu_head_on_stack(), this function has no effect for 294 * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. 295 */ 296 void destroy_rcu_head_on_stack(struct rcu_head *head) 297 { 298 debug_object_free(head, &rcuhead_debug_descr); 299 } 300 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack); 301 302 struct debug_obj_descr rcuhead_debug_descr = { 303 .name = "rcu_head", 304 .fixup_activate = rcuhead_fixup_activate, 305 }; 306 EXPORT_SYMBOL_GPL(rcuhead_debug_descr); 307 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */ 308 309 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE) 310 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp, 311 unsigned long secs, 312 unsigned long c_old, unsigned long c) 313 { 314 trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c); 315 } 316 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read); 317 #else 318 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \ 319 do { } while (0) 320 #endif 321 322 #ifdef CONFIG_RCU_STALL_COMMON 323 324 #ifdef CONFIG_PROVE_RCU 325 #define RCU_STALL_DELAY_DELTA (5 * HZ) 326 #else 327 #define RCU_STALL_DELAY_DELTA 0 328 #endif 329 330 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */ 331 static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT; 332 333 module_param(rcu_cpu_stall_suppress, int, 0644); 334 module_param(rcu_cpu_stall_timeout, int, 0644); 335 336 int rcu_jiffies_till_stall_check(void) 337 { 338 int till_stall_check = ACCESS_ONCE(rcu_cpu_stall_timeout); 339 340 /* 341 * Limit check must be consistent with the Kconfig limits 342 * for CONFIG_RCU_CPU_STALL_TIMEOUT. 343 */ 344 if (till_stall_check < 3) { 345 ACCESS_ONCE(rcu_cpu_stall_timeout) = 3; 346 till_stall_check = 3; 347 } else if (till_stall_check > 300) { 348 ACCESS_ONCE(rcu_cpu_stall_timeout) = 300; 349 till_stall_check = 300; 350 } 351 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA; 352 } 353 354 void rcu_sysrq_start(void) 355 { 356 if (!rcu_cpu_stall_suppress) 357 rcu_cpu_stall_suppress = 2; 358 } 359 360 void rcu_sysrq_end(void) 361 { 362 if (rcu_cpu_stall_suppress == 2) 363 rcu_cpu_stall_suppress = 0; 364 } 365 366 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr) 367 { 368 rcu_cpu_stall_suppress = 1; 369 return NOTIFY_DONE; 370 } 371 372 static struct notifier_block rcu_panic_block = { 373 .notifier_call = rcu_panic, 374 }; 375 376 static int __init check_cpu_stall_init(void) 377 { 378 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block); 379 return 0; 380 } 381 early_initcall(check_cpu_stall_init); 382 383 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */ 384 385 #ifdef CONFIG_TASKS_RCU 386 387 /* 388 * Simple variant of RCU whose quiescent states are voluntary context switch, 389 * user-space execution, and idle. As such, grace periods can take one good 390 * long time. There are no read-side primitives similar to rcu_read_lock() 391 * and rcu_read_unlock() because this implementation is intended to get 392 * the system into a safe state for some of the manipulations involved in 393 * tracing and the like. Finally, this implementation does not support 394 * high call_rcu_tasks() rates from multiple CPUs. If this is required, 395 * per-CPU callback lists will be needed. 396 */ 397 398 /* Global list of callbacks and associated lock. */ 399 static struct rcu_head *rcu_tasks_cbs_head; 400 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; 401 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq); 402 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock); 403 404 /* Track exiting tasks in order to allow them to be waited for. */ 405 DEFINE_SRCU(tasks_rcu_exit_srcu); 406 407 /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */ 408 static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10; 409 module_param(rcu_task_stall_timeout, int, 0644); 410 411 static void rcu_spawn_tasks_kthread(void); 412 413 /* 414 * Post an RCU-tasks callback. First call must be from process context 415 * after the scheduler if fully operational. 416 */ 417 void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp)) 418 { 419 unsigned long flags; 420 bool needwake; 421 422 rhp->next = NULL; 423 rhp->func = func; 424 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); 425 needwake = !rcu_tasks_cbs_head; 426 *rcu_tasks_cbs_tail = rhp; 427 rcu_tasks_cbs_tail = &rhp->next; 428 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); 429 if (needwake) { 430 rcu_spawn_tasks_kthread(); 431 wake_up(&rcu_tasks_cbs_wq); 432 } 433 } 434 EXPORT_SYMBOL_GPL(call_rcu_tasks); 435 436 /** 437 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed. 438 * 439 * Control will return to the caller some time after a full rcu-tasks 440 * grace period has elapsed, in other words after all currently 441 * executing rcu-tasks read-side critical sections have elapsed. These 442 * read-side critical sections are delimited by calls to schedule(), 443 * cond_resched_rcu_qs(), idle execution, userspace execution, calls 444 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched(). 445 * 446 * This is a very specialized primitive, intended only for a few uses in 447 * tracing and other situations requiring manipulation of function 448 * preambles and profiling hooks. The synchronize_rcu_tasks() function 449 * is not (yet) intended for heavy use from multiple CPUs. 450 * 451 * Note that this guarantee implies further memory-ordering guarantees. 452 * On systems with more than one CPU, when synchronize_rcu_tasks() returns, 453 * each CPU is guaranteed to have executed a full memory barrier since the 454 * end of its last RCU-tasks read-side critical section whose beginning 455 * preceded the call to synchronize_rcu_tasks(). In addition, each CPU 456 * having an RCU-tasks read-side critical section that extends beyond 457 * the return from synchronize_rcu_tasks() is guaranteed to have executed 458 * a full memory barrier after the beginning of synchronize_rcu_tasks() 459 * and before the beginning of that RCU-tasks read-side critical section. 460 * Note that these guarantees include CPUs that are offline, idle, or 461 * executing in user mode, as well as CPUs that are executing in the kernel. 462 * 463 * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned 464 * to its caller on CPU B, then both CPU A and CPU B are guaranteed 465 * to have executed a full memory barrier during the execution of 466 * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU 467 * (but again only if the system has more than one CPU). 468 */ 469 void synchronize_rcu_tasks(void) 470 { 471 /* Complain if the scheduler has not started. */ 472 rcu_lockdep_assert(!rcu_scheduler_active, 473 "synchronize_rcu_tasks called too soon"); 474 475 /* Wait for the grace period. */ 476 wait_rcu_gp(call_rcu_tasks); 477 } 478 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks); 479 480 /** 481 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks. 482 * 483 * Although the current implementation is guaranteed to wait, it is not 484 * obligated to, for example, if there are no pending callbacks. 485 */ 486 void rcu_barrier_tasks(void) 487 { 488 /* There is only one callback queue, so this is easy. ;-) */ 489 synchronize_rcu_tasks(); 490 } 491 EXPORT_SYMBOL_GPL(rcu_barrier_tasks); 492 493 /* See if tasks are still holding out, complain if so. */ 494 static void check_holdout_task(struct task_struct *t, 495 bool needreport, bool *firstreport) 496 { 497 int cpu; 498 499 if (!ACCESS_ONCE(t->rcu_tasks_holdout) || 500 t->rcu_tasks_nvcsw != ACCESS_ONCE(t->nvcsw) || 501 !ACCESS_ONCE(t->on_rq) || 502 (IS_ENABLED(CONFIG_NO_HZ_FULL) && 503 !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) { 504 ACCESS_ONCE(t->rcu_tasks_holdout) = false; 505 list_del_init(&t->rcu_tasks_holdout_list); 506 put_task_struct(t); 507 return; 508 } 509 if (!needreport) 510 return; 511 if (*firstreport) { 512 pr_err("INFO: rcu_tasks detected stalls on tasks:\n"); 513 *firstreport = false; 514 } 515 cpu = task_cpu(t); 516 pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n", 517 t, ".I"[is_idle_task(t)], 518 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)], 519 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout, 520 t->rcu_tasks_idle_cpu, cpu); 521 sched_show_task(t); 522 } 523 524 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */ 525 static int __noreturn rcu_tasks_kthread(void *arg) 526 { 527 unsigned long flags; 528 struct task_struct *g, *t; 529 unsigned long lastreport; 530 struct rcu_head *list; 531 struct rcu_head *next; 532 LIST_HEAD(rcu_tasks_holdouts); 533 534 /* Run on housekeeping CPUs by default. Sysadm can move if desired. */ 535 housekeeping_affine(current); 536 537 /* 538 * Each pass through the following loop makes one check for 539 * newly arrived callbacks, and, if there are some, waits for 540 * one RCU-tasks grace period and then invokes the callbacks. 541 * This loop is terminated by the system going down. ;-) 542 */ 543 for (;;) { 544 545 /* Pick up any new callbacks. */ 546 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); 547 list = rcu_tasks_cbs_head; 548 rcu_tasks_cbs_head = NULL; 549 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; 550 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); 551 552 /* If there were none, wait a bit and start over. */ 553 if (!list) { 554 wait_event_interruptible(rcu_tasks_cbs_wq, 555 rcu_tasks_cbs_head); 556 if (!rcu_tasks_cbs_head) { 557 WARN_ON(signal_pending(current)); 558 schedule_timeout_interruptible(HZ/10); 559 } 560 continue; 561 } 562 563 /* 564 * Wait for all pre-existing t->on_rq and t->nvcsw 565 * transitions to complete. Invoking synchronize_sched() 566 * suffices because all these transitions occur with 567 * interrupts disabled. Without this synchronize_sched(), 568 * a read-side critical section that started before the 569 * grace period might be incorrectly seen as having started 570 * after the grace period. 571 * 572 * This synchronize_sched() also dispenses with the 573 * need for a memory barrier on the first store to 574 * ->rcu_tasks_holdout, as it forces the store to happen 575 * after the beginning of the grace period. 576 */ 577 synchronize_sched(); 578 579 /* 580 * There were callbacks, so we need to wait for an 581 * RCU-tasks grace period. Start off by scanning 582 * the task list for tasks that are not already 583 * voluntarily blocked. Mark these tasks and make 584 * a list of them in rcu_tasks_holdouts. 585 */ 586 rcu_read_lock(); 587 for_each_process_thread(g, t) { 588 if (t != current && ACCESS_ONCE(t->on_rq) && 589 !is_idle_task(t)) { 590 get_task_struct(t); 591 t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw); 592 ACCESS_ONCE(t->rcu_tasks_holdout) = true; 593 list_add(&t->rcu_tasks_holdout_list, 594 &rcu_tasks_holdouts); 595 } 596 } 597 rcu_read_unlock(); 598 599 /* 600 * Wait for tasks that are in the process of exiting. 601 * This does only part of the job, ensuring that all 602 * tasks that were previously exiting reach the point 603 * where they have disabled preemption, allowing the 604 * later synchronize_sched() to finish the job. 605 */ 606 synchronize_srcu(&tasks_rcu_exit_srcu); 607 608 /* 609 * Each pass through the following loop scans the list 610 * of holdout tasks, removing any that are no longer 611 * holdouts. When the list is empty, we are done. 612 */ 613 lastreport = jiffies; 614 while (!list_empty(&rcu_tasks_holdouts)) { 615 bool firstreport; 616 bool needreport; 617 int rtst; 618 struct task_struct *t1; 619 620 schedule_timeout_interruptible(HZ); 621 rtst = ACCESS_ONCE(rcu_task_stall_timeout); 622 needreport = rtst > 0 && 623 time_after(jiffies, lastreport + rtst); 624 if (needreport) 625 lastreport = jiffies; 626 firstreport = true; 627 WARN_ON(signal_pending(current)); 628 list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts, 629 rcu_tasks_holdout_list) { 630 check_holdout_task(t, needreport, &firstreport); 631 cond_resched(); 632 } 633 } 634 635 /* 636 * Because ->on_rq and ->nvcsw are not guaranteed 637 * to have a full memory barriers prior to them in the 638 * schedule() path, memory reordering on other CPUs could 639 * cause their RCU-tasks read-side critical sections to 640 * extend past the end of the grace period. However, 641 * because these ->nvcsw updates are carried out with 642 * interrupts disabled, we can use synchronize_sched() 643 * to force the needed ordering on all such CPUs. 644 * 645 * This synchronize_sched() also confines all 646 * ->rcu_tasks_holdout accesses to be within the grace 647 * period, avoiding the need for memory barriers for 648 * ->rcu_tasks_holdout accesses. 649 * 650 * In addition, this synchronize_sched() waits for exiting 651 * tasks to complete their final preempt_disable() region 652 * of execution, cleaning up after the synchronize_srcu() 653 * above. 654 */ 655 synchronize_sched(); 656 657 /* Invoke the callbacks. */ 658 while (list) { 659 next = list->next; 660 local_bh_disable(); 661 list->func(list); 662 local_bh_enable(); 663 list = next; 664 cond_resched(); 665 } 666 schedule_timeout_uninterruptible(HZ/10); 667 } 668 } 669 670 /* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */ 671 static void rcu_spawn_tasks_kthread(void) 672 { 673 static DEFINE_MUTEX(rcu_tasks_kthread_mutex); 674 static struct task_struct *rcu_tasks_kthread_ptr; 675 struct task_struct *t; 676 677 if (ACCESS_ONCE(rcu_tasks_kthread_ptr)) { 678 smp_mb(); /* Ensure caller sees full kthread. */ 679 return; 680 } 681 mutex_lock(&rcu_tasks_kthread_mutex); 682 if (rcu_tasks_kthread_ptr) { 683 mutex_unlock(&rcu_tasks_kthread_mutex); 684 return; 685 } 686 t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread"); 687 BUG_ON(IS_ERR(t)); 688 smp_mb(); /* Ensure others see full kthread. */ 689 ACCESS_ONCE(rcu_tasks_kthread_ptr) = t; 690 mutex_unlock(&rcu_tasks_kthread_mutex); 691 } 692 693 #endif /* #ifdef CONFIG_TASKS_RCU */ 694 695 #ifdef CONFIG_PROVE_RCU 696 697 /* 698 * Early boot self test parameters, one for each flavor 699 */ 700 static bool rcu_self_test; 701 static bool rcu_self_test_bh; 702 static bool rcu_self_test_sched; 703 704 module_param(rcu_self_test, bool, 0444); 705 module_param(rcu_self_test_bh, bool, 0444); 706 module_param(rcu_self_test_sched, bool, 0444); 707 708 static int rcu_self_test_counter; 709 710 static void test_callback(struct rcu_head *r) 711 { 712 rcu_self_test_counter++; 713 pr_info("RCU test callback executed %d\n", rcu_self_test_counter); 714 } 715 716 static void early_boot_test_call_rcu(void) 717 { 718 static struct rcu_head head; 719 720 call_rcu(&head, test_callback); 721 } 722 723 static void early_boot_test_call_rcu_bh(void) 724 { 725 static struct rcu_head head; 726 727 call_rcu_bh(&head, test_callback); 728 } 729 730 static void early_boot_test_call_rcu_sched(void) 731 { 732 static struct rcu_head head; 733 734 call_rcu_sched(&head, test_callback); 735 } 736 737 void rcu_early_boot_tests(void) 738 { 739 pr_info("Running RCU self tests\n"); 740 741 if (rcu_self_test) 742 early_boot_test_call_rcu(); 743 if (rcu_self_test_bh) 744 early_boot_test_call_rcu_bh(); 745 if (rcu_self_test_sched) 746 early_boot_test_call_rcu_sched(); 747 } 748 749 static int rcu_verify_early_boot_tests(void) 750 { 751 int ret = 0; 752 int early_boot_test_counter = 0; 753 754 if (rcu_self_test) { 755 early_boot_test_counter++; 756 rcu_barrier(); 757 } 758 if (rcu_self_test_bh) { 759 early_boot_test_counter++; 760 rcu_barrier_bh(); 761 } 762 if (rcu_self_test_sched) { 763 early_boot_test_counter++; 764 rcu_barrier_sched(); 765 } 766 767 if (rcu_self_test_counter != early_boot_test_counter) { 768 WARN_ON(1); 769 ret = -1; 770 } 771 772 return ret; 773 } 774 late_initcall(rcu_verify_early_boot_tests); 775 #else 776 void rcu_early_boot_tests(void) {} 777 #endif /* CONFIG_PROVE_RCU */ 778