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