1 /* 2 * Read-Copy Update mechanism for mutual exclusion 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License as published by 6 * the Free Software Foundation; either version 2 of the License, or 7 * (at your option) any later version. 8 * 9 * This program is distributed in the hope that it will be useful, 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 * GNU General Public License for more details. 13 * 14 * You should have received a copy of the GNU General Public License 15 * along with this program; if not, you can access it online at 16 * http://www.gnu.org/licenses/gpl-2.0.html. 17 * 18 * Copyright IBM Corporation, 2001 19 * 20 * Authors: Dipankar Sarma <dipankar@in.ibm.com> 21 * Manfred Spraul <manfred@colorfullife.com> 22 * 23 * Based on the original work by Paul McKenney <paulmck@us.ibm.com> 24 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen. 25 * Papers: 26 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf 27 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001) 28 * 29 * For detailed explanation of Read-Copy Update mechanism see - 30 * http://lse.sourceforge.net/locking/rcupdate.html 31 * 32 */ 33 #include <linux/types.h> 34 #include <linux/kernel.h> 35 #include <linux/init.h> 36 #include <linux/spinlock.h> 37 #include <linux/smp.h> 38 #include <linux/interrupt.h> 39 #include <linux/sched/signal.h> 40 #include <linux/sched/debug.h> 41 #include <linux/atomic.h> 42 #include <linux/bitops.h> 43 #include <linux/percpu.h> 44 #include <linux/notifier.h> 45 #include <linux/cpu.h> 46 #include <linux/mutex.h> 47 #include <linux/export.h> 48 #include <linux/hardirq.h> 49 #include <linux/delay.h> 50 #include <linux/moduleparam.h> 51 #include <linux/kthread.h> 52 #include <linux/tick.h> 53 #include <linux/rcupdate_wait.h> 54 55 #define CREATE_TRACE_POINTS 56 57 #include "rcu.h" 58 59 #ifdef MODULE_PARAM_PREFIX 60 #undef MODULE_PARAM_PREFIX 61 #endif 62 #define MODULE_PARAM_PREFIX "rcupdate." 63 64 #ifndef CONFIG_TINY_RCU 65 extern int rcu_expedited; /* from sysctl */ 66 module_param(rcu_expedited, int, 0); 67 extern int rcu_normal; /* from sysctl */ 68 module_param(rcu_normal, int, 0); 69 static int rcu_normal_after_boot; 70 module_param(rcu_normal_after_boot, int, 0); 71 #endif /* #ifndef CONFIG_TINY_RCU */ 72 73 #ifdef CONFIG_DEBUG_LOCK_ALLOC 74 /** 75 * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section? 76 * 77 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an 78 * RCU-sched read-side critical section. In absence of 79 * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side 80 * critical section unless it can prove otherwise. Note that disabling 81 * of preemption (including disabling irqs) counts as an RCU-sched 82 * read-side critical section. This is useful for debug checks in functions 83 * that required that they be called within an RCU-sched read-side 84 * critical section. 85 * 86 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot 87 * and while lockdep is disabled. 88 * 89 * Note that if the CPU is in the idle loop from an RCU point of 90 * view (ie: that we are in the section between rcu_idle_enter() and 91 * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU 92 * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs 93 * that are in such a section, considering these as in extended quiescent 94 * state, so such a CPU is effectively never in an RCU read-side critical 95 * section regardless of what RCU primitives it invokes. This state of 96 * affairs is required --- we need to keep an RCU-free window in idle 97 * where the CPU may possibly enter into low power mode. This way we can 98 * notice an extended quiescent state to other CPUs that started a grace 99 * period. Otherwise we would delay any grace period as long as we run in 100 * the idle task. 101 * 102 * Similarly, we avoid claiming an SRCU read lock held if the current 103 * CPU is offline. 104 */ 105 int rcu_read_lock_sched_held(void) 106 { 107 int lockdep_opinion = 0; 108 109 if (!debug_lockdep_rcu_enabled()) 110 return 1; 111 if (!rcu_is_watching()) 112 return 0; 113 if (!rcu_lockdep_current_cpu_online()) 114 return 0; 115 if (debug_locks) 116 lockdep_opinion = lock_is_held(&rcu_sched_lock_map); 117 return lockdep_opinion || !preemptible(); 118 } 119 EXPORT_SYMBOL(rcu_read_lock_sched_held); 120 #endif 121 122 #ifndef CONFIG_TINY_RCU 123 124 /* 125 * Should expedited grace-period primitives always fall back to their 126 * non-expedited counterparts? Intended for use within RCU. Note 127 * that if the user specifies both rcu_expedited and rcu_normal, then 128 * rcu_normal wins. (Except during the time period during boot from 129 * when the first task is spawned until the rcu_set_runtime_mode() 130 * core_initcall() is invoked, at which point everything is expedited.) 131 */ 132 bool rcu_gp_is_normal(void) 133 { 134 return READ_ONCE(rcu_normal) && 135 rcu_scheduler_active != RCU_SCHEDULER_INIT; 136 } 137 EXPORT_SYMBOL_GPL(rcu_gp_is_normal); 138 139 static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1); 140 141 /* 142 * Should normal grace-period primitives be expedited? Intended for 143 * use within RCU. Note that this function takes the rcu_expedited 144 * sysfs/boot variable and rcu_scheduler_active into account as well 145 * as the rcu_expedite_gp() nesting. So looping on rcu_unexpedite_gp() 146 * until rcu_gp_is_expedited() returns false is a -really- bad idea. 147 */ 148 bool rcu_gp_is_expedited(void) 149 { 150 return rcu_expedited || atomic_read(&rcu_expedited_nesting) || 151 rcu_scheduler_active == RCU_SCHEDULER_INIT; 152 } 153 EXPORT_SYMBOL_GPL(rcu_gp_is_expedited); 154 155 /** 156 * rcu_expedite_gp - Expedite future RCU grace periods 157 * 158 * After a call to this function, future calls to synchronize_rcu() and 159 * friends act as the corresponding synchronize_rcu_expedited() function 160 * had instead been called. 161 */ 162 void rcu_expedite_gp(void) 163 { 164 atomic_inc(&rcu_expedited_nesting); 165 } 166 EXPORT_SYMBOL_GPL(rcu_expedite_gp); 167 168 /** 169 * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation 170 * 171 * Undo a prior call to rcu_expedite_gp(). If all prior calls to 172 * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(), 173 * and if the rcu_expedited sysfs/boot parameter is not set, then all 174 * subsequent calls to synchronize_rcu() and friends will return to 175 * their normal non-expedited behavior. 176 */ 177 void rcu_unexpedite_gp(void) 178 { 179 atomic_dec(&rcu_expedited_nesting); 180 } 181 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp); 182 183 /* 184 * Inform RCU of the end of the in-kernel boot sequence. 185 */ 186 void rcu_end_inkernel_boot(void) 187 { 188 rcu_unexpedite_gp(); 189 if (rcu_normal_after_boot) 190 WRITE_ONCE(rcu_normal, 1); 191 } 192 193 #endif /* #ifndef CONFIG_TINY_RCU */ 194 195 /* 196 * Test each non-SRCU synchronous grace-period wait API. This is 197 * useful just after a change in mode for these primitives, and 198 * during early boot. 199 */ 200 void rcu_test_sync_prims(void) 201 { 202 if (!IS_ENABLED(CONFIG_PROVE_RCU)) 203 return; 204 synchronize_rcu(); 205 synchronize_rcu_bh(); 206 synchronize_sched(); 207 synchronize_rcu_expedited(); 208 synchronize_rcu_bh_expedited(); 209 synchronize_sched_expedited(); 210 } 211 212 #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) 213 214 /* 215 * Switch to run-time mode once RCU has fully initialized. 216 */ 217 static int __init rcu_set_runtime_mode(void) 218 { 219 rcu_test_sync_prims(); 220 rcu_scheduler_active = RCU_SCHEDULER_RUNNING; 221 rcu_test_sync_prims(); 222 return 0; 223 } 224 core_initcall(rcu_set_runtime_mode); 225 226 #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */ 227 228 #ifdef CONFIG_PREEMPT_RCU 229 230 /* 231 * Preemptible RCU implementation for rcu_read_lock(). 232 * Just increment ->rcu_read_lock_nesting, shared state will be updated 233 * if we block. 234 */ 235 void __rcu_read_lock(void) 236 { 237 current->rcu_read_lock_nesting++; 238 barrier(); /* critical section after entry code. */ 239 } 240 EXPORT_SYMBOL_GPL(__rcu_read_lock); 241 242 /* 243 * Preemptible RCU implementation for rcu_read_unlock(). 244 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost 245 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then 246 * invoke rcu_read_unlock_special() to clean up after a context switch 247 * in an RCU read-side critical section and other special cases. 248 */ 249 void __rcu_read_unlock(void) 250 { 251 struct task_struct *t = current; 252 253 if (t->rcu_read_lock_nesting != 1) { 254 --t->rcu_read_lock_nesting; 255 } else { 256 barrier(); /* critical section before exit code. */ 257 t->rcu_read_lock_nesting = INT_MIN; 258 barrier(); /* assign before ->rcu_read_unlock_special load */ 259 if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) 260 rcu_read_unlock_special(t); 261 barrier(); /* ->rcu_read_unlock_special load before assign */ 262 t->rcu_read_lock_nesting = 0; 263 } 264 #ifdef CONFIG_PROVE_LOCKING 265 { 266 int rrln = READ_ONCE(t->rcu_read_lock_nesting); 267 268 WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2); 269 } 270 #endif /* #ifdef CONFIG_PROVE_LOCKING */ 271 } 272 EXPORT_SYMBOL_GPL(__rcu_read_unlock); 273 274 #endif /* #ifdef CONFIG_PREEMPT_RCU */ 275 276 #ifdef CONFIG_DEBUG_LOCK_ALLOC 277 static struct lock_class_key rcu_lock_key; 278 struct lockdep_map rcu_lock_map = 279 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key); 280 EXPORT_SYMBOL_GPL(rcu_lock_map); 281 282 static struct lock_class_key rcu_bh_lock_key; 283 struct lockdep_map rcu_bh_lock_map = 284 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key); 285 EXPORT_SYMBOL_GPL(rcu_bh_lock_map); 286 287 static struct lock_class_key rcu_sched_lock_key; 288 struct lockdep_map rcu_sched_lock_map = 289 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key); 290 EXPORT_SYMBOL_GPL(rcu_sched_lock_map); 291 292 static struct lock_class_key rcu_callback_key; 293 struct lockdep_map rcu_callback_map = 294 STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key); 295 EXPORT_SYMBOL_GPL(rcu_callback_map); 296 297 int notrace debug_lockdep_rcu_enabled(void) 298 { 299 return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && 300 current->lockdep_recursion == 0; 301 } 302 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled); 303 304 /** 305 * rcu_read_lock_held() - might we be in RCU read-side critical section? 306 * 307 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU 308 * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC, 309 * this assumes we are in an RCU read-side critical section unless it can 310 * prove otherwise. This is useful for debug checks in functions that 311 * require that they be called within an RCU read-side critical section. 312 * 313 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot 314 * and while lockdep is disabled. 315 * 316 * Note that rcu_read_lock() and the matching rcu_read_unlock() must 317 * occur in the same context, for example, it is illegal to invoke 318 * rcu_read_unlock() in process context if the matching rcu_read_lock() 319 * was invoked from within an irq handler. 320 * 321 * Note that rcu_read_lock() is disallowed if the CPU is either idle or 322 * offline from an RCU perspective, so check for those as well. 323 */ 324 int rcu_read_lock_held(void) 325 { 326 if (!debug_lockdep_rcu_enabled()) 327 return 1; 328 if (!rcu_is_watching()) 329 return 0; 330 if (!rcu_lockdep_current_cpu_online()) 331 return 0; 332 return lock_is_held(&rcu_lock_map); 333 } 334 EXPORT_SYMBOL_GPL(rcu_read_lock_held); 335 336 /** 337 * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section? 338 * 339 * Check for bottom half being disabled, which covers both the 340 * CONFIG_PROVE_RCU and not cases. Note that if someone uses 341 * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled) 342 * will show the situation. This is useful for debug checks in functions 343 * that require that they be called within an RCU read-side critical 344 * section. 345 * 346 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot. 347 * 348 * Note that rcu_read_lock() is disallowed if the CPU is either idle or 349 * offline from an RCU perspective, so check for those as well. 350 */ 351 int rcu_read_lock_bh_held(void) 352 { 353 if (!debug_lockdep_rcu_enabled()) 354 return 1; 355 if (!rcu_is_watching()) 356 return 0; 357 if (!rcu_lockdep_current_cpu_online()) 358 return 0; 359 return in_softirq() || irqs_disabled(); 360 } 361 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held); 362 363 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ 364 365 /** 366 * wakeme_after_rcu() - Callback function to awaken a task after grace period 367 * @head: Pointer to rcu_head member within rcu_synchronize structure 368 * 369 * Awaken the corresponding task now that a grace period has elapsed. 370 */ 371 void wakeme_after_rcu(struct rcu_head *head) 372 { 373 struct rcu_synchronize *rcu; 374 375 rcu = container_of(head, struct rcu_synchronize, head); 376 complete(&rcu->completion); 377 } 378 EXPORT_SYMBOL_GPL(wakeme_after_rcu); 379 380 void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array, 381 struct rcu_synchronize *rs_array) 382 { 383 int i; 384 int j; 385 386 /* Initialize and register callbacks for each flavor specified. */ 387 for (i = 0; i < n; i++) { 388 if (checktiny && 389 (crcu_array[i] == call_rcu || 390 crcu_array[i] == call_rcu_bh)) { 391 might_sleep(); 392 continue; 393 } 394 init_rcu_head_on_stack(&rs_array[i].head); 395 init_completion(&rs_array[i].completion); 396 for (j = 0; j < i; j++) 397 if (crcu_array[j] == crcu_array[i]) 398 break; 399 if (j == i) 400 (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu); 401 } 402 403 /* Wait for all callbacks to be invoked. */ 404 for (i = 0; i < n; i++) { 405 if (checktiny && 406 (crcu_array[i] == call_rcu || 407 crcu_array[i] == call_rcu_bh)) 408 continue; 409 for (j = 0; j < i; j++) 410 if (crcu_array[j] == crcu_array[i]) 411 break; 412 if (j == i) 413 wait_for_completion(&rs_array[i].completion); 414 destroy_rcu_head_on_stack(&rs_array[i].head); 415 } 416 } 417 EXPORT_SYMBOL_GPL(__wait_rcu_gp); 418 419 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD 420 void init_rcu_head(struct rcu_head *head) 421 { 422 debug_object_init(head, &rcuhead_debug_descr); 423 } 424 425 void destroy_rcu_head(struct rcu_head *head) 426 { 427 debug_object_free(head, &rcuhead_debug_descr); 428 } 429 430 static bool rcuhead_is_static_object(void *addr) 431 { 432 return true; 433 } 434 435 /** 436 * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects 437 * @head: pointer to rcu_head structure to be initialized 438 * 439 * This function informs debugobjects of a new rcu_head structure that 440 * has been allocated as an auto variable on the stack. This function 441 * is not required for rcu_head structures that are statically defined or 442 * that are dynamically allocated on the heap. This function has no 443 * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. 444 */ 445 void init_rcu_head_on_stack(struct rcu_head *head) 446 { 447 debug_object_init_on_stack(head, &rcuhead_debug_descr); 448 } 449 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack); 450 451 /** 452 * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects 453 * @head: pointer to rcu_head structure to be initialized 454 * 455 * This function informs debugobjects that an on-stack rcu_head structure 456 * is about to go out of scope. As with init_rcu_head_on_stack(), this 457 * function is not required for rcu_head structures that are statically 458 * defined or that are dynamically allocated on the heap. Also as with 459 * init_rcu_head_on_stack(), this function has no effect for 460 * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. 461 */ 462 void destroy_rcu_head_on_stack(struct rcu_head *head) 463 { 464 debug_object_free(head, &rcuhead_debug_descr); 465 } 466 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack); 467 468 struct debug_obj_descr rcuhead_debug_descr = { 469 .name = "rcu_head", 470 .is_static_object = rcuhead_is_static_object, 471 }; 472 EXPORT_SYMBOL_GPL(rcuhead_debug_descr); 473 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */ 474 475 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE) 476 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp, 477 unsigned long secs, 478 unsigned long c_old, unsigned long c) 479 { 480 trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c); 481 } 482 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read); 483 #else 484 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \ 485 do { } while (0) 486 #endif 487 488 #ifdef CONFIG_RCU_STALL_COMMON 489 490 #ifdef CONFIG_PROVE_RCU 491 #define RCU_STALL_DELAY_DELTA (5 * HZ) 492 #else 493 #define RCU_STALL_DELAY_DELTA 0 494 #endif 495 496 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */ 497 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress); 498 static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT; 499 500 module_param(rcu_cpu_stall_suppress, int, 0644); 501 module_param(rcu_cpu_stall_timeout, int, 0644); 502 503 int rcu_jiffies_till_stall_check(void) 504 { 505 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout); 506 507 /* 508 * Limit check must be consistent with the Kconfig limits 509 * for CONFIG_RCU_CPU_STALL_TIMEOUT. 510 */ 511 if (till_stall_check < 3) { 512 WRITE_ONCE(rcu_cpu_stall_timeout, 3); 513 till_stall_check = 3; 514 } else if (till_stall_check > 300) { 515 WRITE_ONCE(rcu_cpu_stall_timeout, 300); 516 till_stall_check = 300; 517 } 518 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA; 519 } 520 521 void rcu_sysrq_start(void) 522 { 523 if (!rcu_cpu_stall_suppress) 524 rcu_cpu_stall_suppress = 2; 525 } 526 527 void rcu_sysrq_end(void) 528 { 529 if (rcu_cpu_stall_suppress == 2) 530 rcu_cpu_stall_suppress = 0; 531 } 532 533 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr) 534 { 535 rcu_cpu_stall_suppress = 1; 536 return NOTIFY_DONE; 537 } 538 539 static struct notifier_block rcu_panic_block = { 540 .notifier_call = rcu_panic, 541 }; 542 543 static int __init check_cpu_stall_init(void) 544 { 545 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block); 546 return 0; 547 } 548 early_initcall(check_cpu_stall_init); 549 550 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */ 551 552 #ifdef CONFIG_TASKS_RCU 553 554 /* 555 * Simple variant of RCU whose quiescent states are voluntary context switch, 556 * user-space execution, and idle. As such, grace periods can take one good 557 * long time. There are no read-side primitives similar to rcu_read_lock() 558 * and rcu_read_unlock() because this implementation is intended to get 559 * the system into a safe state for some of the manipulations involved in 560 * tracing and the like. Finally, this implementation does not support 561 * high call_rcu_tasks() rates from multiple CPUs. If this is required, 562 * per-CPU callback lists will be needed. 563 */ 564 565 /* Global list of callbacks and associated lock. */ 566 static struct rcu_head *rcu_tasks_cbs_head; 567 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; 568 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq); 569 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock); 570 571 /* Track exiting tasks in order to allow them to be waited for. */ 572 DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu); 573 574 /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */ 575 #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10) 576 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT; 577 module_param(rcu_task_stall_timeout, int, 0644); 578 579 static struct task_struct *rcu_tasks_kthread_ptr; 580 581 /** 582 * call_rcu_tasks() - Queue an RCU for invocation task-based grace period 583 * @rhp: structure to be used for queueing the RCU updates. 584 * @func: actual callback function to be invoked after the grace period 585 * 586 * The callback function will be invoked some time after a full grace 587 * period elapses, in other words after all currently executing RCU 588 * read-side critical sections have completed. call_rcu_tasks() assumes 589 * that the read-side critical sections end at a voluntary context 590 * switch (not a preemption!), entry into idle, or transition to usermode 591 * execution. As such, there are no read-side primitives analogous to 592 * rcu_read_lock() and rcu_read_unlock() because this primitive is intended 593 * to determine that all tasks have passed through a safe state, not so 594 * much for data-strcuture synchronization. 595 * 596 * See the description of call_rcu() for more detailed information on 597 * memory ordering guarantees. 598 */ 599 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func) 600 { 601 unsigned long flags; 602 bool needwake; 603 604 rhp->next = NULL; 605 rhp->func = func; 606 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); 607 needwake = !rcu_tasks_cbs_head; 608 *rcu_tasks_cbs_tail = rhp; 609 rcu_tasks_cbs_tail = &rhp->next; 610 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); 611 /* We can't create the thread unless interrupts are enabled. */ 612 if (needwake && READ_ONCE(rcu_tasks_kthread_ptr)) 613 wake_up(&rcu_tasks_cbs_wq); 614 } 615 EXPORT_SYMBOL_GPL(call_rcu_tasks); 616 617 /** 618 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed. 619 * 620 * Control will return to the caller some time after a full rcu-tasks 621 * grace period has elapsed, in other words after all currently 622 * executing rcu-tasks read-side critical sections have elapsed. These 623 * read-side critical sections are delimited by calls to schedule(), 624 * cond_resched_rcu_qs(), idle execution, userspace execution, calls 625 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched(). 626 * 627 * This is a very specialized primitive, intended only for a few uses in 628 * tracing and other situations requiring manipulation of function 629 * preambles and profiling hooks. The synchronize_rcu_tasks() function 630 * is not (yet) intended for heavy use from multiple CPUs. 631 * 632 * Note that this guarantee implies further memory-ordering guarantees. 633 * On systems with more than one CPU, when synchronize_rcu_tasks() returns, 634 * each CPU is guaranteed to have executed a full memory barrier since the 635 * end of its last RCU-tasks read-side critical section whose beginning 636 * preceded the call to synchronize_rcu_tasks(). In addition, each CPU 637 * having an RCU-tasks read-side critical section that extends beyond 638 * the return from synchronize_rcu_tasks() is guaranteed to have executed 639 * a full memory barrier after the beginning of synchronize_rcu_tasks() 640 * and before the beginning of that RCU-tasks read-side critical section. 641 * Note that these guarantees include CPUs that are offline, idle, or 642 * executing in user mode, as well as CPUs that are executing in the kernel. 643 * 644 * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned 645 * to its caller on CPU B, then both CPU A and CPU B are guaranteed 646 * to have executed a full memory barrier during the execution of 647 * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU 648 * (but again only if the system has more than one CPU). 649 */ 650 void synchronize_rcu_tasks(void) 651 { 652 /* Complain if the scheduler has not started. */ 653 RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE, 654 "synchronize_rcu_tasks called too soon"); 655 656 /* Wait for the grace period. */ 657 wait_rcu_gp(call_rcu_tasks); 658 } 659 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks); 660 661 /** 662 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks. 663 * 664 * Although the current implementation is guaranteed to wait, it is not 665 * obligated to, for example, if there are no pending callbacks. 666 */ 667 void rcu_barrier_tasks(void) 668 { 669 /* There is only one callback queue, so this is easy. ;-) */ 670 synchronize_rcu_tasks(); 671 } 672 EXPORT_SYMBOL_GPL(rcu_barrier_tasks); 673 674 /* See if tasks are still holding out, complain if so. */ 675 static void check_holdout_task(struct task_struct *t, 676 bool needreport, bool *firstreport) 677 { 678 int cpu; 679 680 if (!READ_ONCE(t->rcu_tasks_holdout) || 681 t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) || 682 !READ_ONCE(t->on_rq) || 683 (IS_ENABLED(CONFIG_NO_HZ_FULL) && 684 !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) { 685 WRITE_ONCE(t->rcu_tasks_holdout, false); 686 list_del_init(&t->rcu_tasks_holdout_list); 687 put_task_struct(t); 688 return; 689 } 690 rcu_request_urgent_qs_task(t); 691 if (!needreport) 692 return; 693 if (*firstreport) { 694 pr_err("INFO: rcu_tasks detected stalls on tasks:\n"); 695 *firstreport = false; 696 } 697 cpu = task_cpu(t); 698 pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n", 699 t, ".I"[is_idle_task(t)], 700 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)], 701 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout, 702 t->rcu_tasks_idle_cpu, cpu); 703 sched_show_task(t); 704 } 705 706 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */ 707 static int __noreturn rcu_tasks_kthread(void *arg) 708 { 709 unsigned long flags; 710 struct task_struct *g, *t; 711 unsigned long lastreport; 712 struct rcu_head *list; 713 struct rcu_head *next; 714 LIST_HEAD(rcu_tasks_holdouts); 715 716 /* Run on housekeeping CPUs by default. Sysadm can move if desired. */ 717 housekeeping_affine(current); 718 719 /* 720 * Each pass through the following loop makes one check for 721 * newly arrived callbacks, and, if there are some, waits for 722 * one RCU-tasks grace period and then invokes the callbacks. 723 * This loop is terminated by the system going down. ;-) 724 */ 725 for (;;) { 726 727 /* Pick up any new callbacks. */ 728 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); 729 list = rcu_tasks_cbs_head; 730 rcu_tasks_cbs_head = NULL; 731 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; 732 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); 733 734 /* If there were none, wait a bit and start over. */ 735 if (!list) { 736 wait_event_interruptible(rcu_tasks_cbs_wq, 737 rcu_tasks_cbs_head); 738 if (!rcu_tasks_cbs_head) { 739 WARN_ON(signal_pending(current)); 740 schedule_timeout_interruptible(HZ/10); 741 } 742 continue; 743 } 744 745 /* 746 * Wait for all pre-existing t->on_rq and t->nvcsw 747 * transitions to complete. Invoking synchronize_sched() 748 * suffices because all these transitions occur with 749 * interrupts disabled. Without this synchronize_sched(), 750 * a read-side critical section that started before the 751 * grace period might be incorrectly seen as having started 752 * after the grace period. 753 * 754 * This synchronize_sched() also dispenses with the 755 * need for a memory barrier on the first store to 756 * ->rcu_tasks_holdout, as it forces the store to happen 757 * after the beginning of the grace period. 758 */ 759 synchronize_sched(); 760 761 /* 762 * There were callbacks, so we need to wait for an 763 * RCU-tasks grace period. Start off by scanning 764 * the task list for tasks that are not already 765 * voluntarily blocked. Mark these tasks and make 766 * a list of them in rcu_tasks_holdouts. 767 */ 768 rcu_read_lock(); 769 for_each_process_thread(g, t) { 770 if (t != current && READ_ONCE(t->on_rq) && 771 !is_idle_task(t)) { 772 get_task_struct(t); 773 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw); 774 WRITE_ONCE(t->rcu_tasks_holdout, true); 775 list_add(&t->rcu_tasks_holdout_list, 776 &rcu_tasks_holdouts); 777 } 778 } 779 rcu_read_unlock(); 780 781 /* 782 * Wait for tasks that are in the process of exiting. 783 * This does only part of the job, ensuring that all 784 * tasks that were previously exiting reach the point 785 * where they have disabled preemption, allowing the 786 * later synchronize_sched() to finish the job. 787 */ 788 synchronize_srcu(&tasks_rcu_exit_srcu); 789 790 /* 791 * Each pass through the following loop scans the list 792 * of holdout tasks, removing any that are no longer 793 * holdouts. When the list is empty, we are done. 794 */ 795 lastreport = jiffies; 796 while (!list_empty(&rcu_tasks_holdouts)) { 797 bool firstreport; 798 bool needreport; 799 int rtst; 800 struct task_struct *t1; 801 802 schedule_timeout_interruptible(HZ); 803 rtst = READ_ONCE(rcu_task_stall_timeout); 804 needreport = rtst > 0 && 805 time_after(jiffies, lastreport + rtst); 806 if (needreport) 807 lastreport = jiffies; 808 firstreport = true; 809 WARN_ON(signal_pending(current)); 810 list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts, 811 rcu_tasks_holdout_list) { 812 check_holdout_task(t, needreport, &firstreport); 813 cond_resched(); 814 } 815 } 816 817 /* 818 * Because ->on_rq and ->nvcsw are not guaranteed 819 * to have a full memory barriers prior to them in the 820 * schedule() path, memory reordering on other CPUs could 821 * cause their RCU-tasks read-side critical sections to 822 * extend past the end of the grace period. However, 823 * because these ->nvcsw updates are carried out with 824 * interrupts disabled, we can use synchronize_sched() 825 * to force the needed ordering on all such CPUs. 826 * 827 * This synchronize_sched() also confines all 828 * ->rcu_tasks_holdout accesses to be within the grace 829 * period, avoiding the need for memory barriers for 830 * ->rcu_tasks_holdout accesses. 831 * 832 * In addition, this synchronize_sched() waits for exiting 833 * tasks to complete their final preempt_disable() region 834 * of execution, cleaning up after the synchronize_srcu() 835 * above. 836 */ 837 synchronize_sched(); 838 839 /* Invoke the callbacks. */ 840 while (list) { 841 next = list->next; 842 local_bh_disable(); 843 list->func(list); 844 local_bh_enable(); 845 list = next; 846 cond_resched(); 847 } 848 schedule_timeout_uninterruptible(HZ/10); 849 } 850 } 851 852 /* Spawn rcu_tasks_kthread() at core_initcall() time. */ 853 static int __init rcu_spawn_tasks_kthread(void) 854 { 855 struct task_struct *t; 856 857 t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread"); 858 BUG_ON(IS_ERR(t)); 859 smp_mb(); /* Ensure others see full kthread. */ 860 WRITE_ONCE(rcu_tasks_kthread_ptr, t); 861 return 0; 862 } 863 core_initcall(rcu_spawn_tasks_kthread); 864 865 /* Do the srcu_read_lock() for the above synchronize_srcu(). */ 866 void exit_tasks_rcu_start(void) 867 { 868 preempt_disable(); 869 current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu); 870 preempt_enable(); 871 } 872 873 /* Do the srcu_read_unlock() for the above synchronize_srcu(). */ 874 void exit_tasks_rcu_finish(void) 875 { 876 preempt_disable(); 877 __srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx); 878 preempt_enable(); 879 } 880 881 #endif /* #ifdef CONFIG_TASKS_RCU */ 882 883 #ifndef CONFIG_TINY_RCU 884 885 /* 886 * Print any non-default Tasks RCU settings. 887 */ 888 static void __init rcu_tasks_bootup_oddness(void) 889 { 890 #ifdef CONFIG_TASKS_RCU 891 if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT) 892 pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout); 893 else 894 pr_info("\tTasks RCU enabled.\n"); 895 #endif /* #ifdef CONFIG_TASKS_RCU */ 896 } 897 898 #endif /* #ifndef CONFIG_TINY_RCU */ 899 900 #ifdef CONFIG_PROVE_RCU 901 902 /* 903 * Early boot self test parameters, one for each flavor 904 */ 905 static bool rcu_self_test; 906 static bool rcu_self_test_bh; 907 static bool rcu_self_test_sched; 908 909 module_param(rcu_self_test, bool, 0444); 910 module_param(rcu_self_test_bh, bool, 0444); 911 module_param(rcu_self_test_sched, bool, 0444); 912 913 static int rcu_self_test_counter; 914 915 static void test_callback(struct rcu_head *r) 916 { 917 rcu_self_test_counter++; 918 pr_info("RCU test callback executed %d\n", rcu_self_test_counter); 919 } 920 921 static void early_boot_test_call_rcu(void) 922 { 923 static struct rcu_head head; 924 925 call_rcu(&head, test_callback); 926 } 927 928 static void early_boot_test_call_rcu_bh(void) 929 { 930 static struct rcu_head head; 931 932 call_rcu_bh(&head, test_callback); 933 } 934 935 static void early_boot_test_call_rcu_sched(void) 936 { 937 static struct rcu_head head; 938 939 call_rcu_sched(&head, test_callback); 940 } 941 942 void rcu_early_boot_tests(void) 943 { 944 pr_info("Running RCU self tests\n"); 945 946 if (rcu_self_test) 947 early_boot_test_call_rcu(); 948 if (rcu_self_test_bh) 949 early_boot_test_call_rcu_bh(); 950 if (rcu_self_test_sched) 951 early_boot_test_call_rcu_sched(); 952 rcu_test_sync_prims(); 953 } 954 955 static int rcu_verify_early_boot_tests(void) 956 { 957 int ret = 0; 958 int early_boot_test_counter = 0; 959 960 if (rcu_self_test) { 961 early_boot_test_counter++; 962 rcu_barrier(); 963 } 964 if (rcu_self_test_bh) { 965 early_boot_test_counter++; 966 rcu_barrier_bh(); 967 } 968 if (rcu_self_test_sched) { 969 early_boot_test_counter++; 970 rcu_barrier_sched(); 971 } 972 973 if (rcu_self_test_counter != early_boot_test_counter) { 974 WARN_ON(1); 975 ret = -1; 976 } 977 978 return ret; 979 } 980 late_initcall(rcu_verify_early_boot_tests); 981 #else 982 void rcu_early_boot_tests(void) {} 983 #endif /* CONFIG_PROVE_RCU */ 984 985 #ifndef CONFIG_TINY_RCU 986 987 /* 988 * Print any significant non-default boot-time settings. 989 */ 990 void __init rcupdate_announce_bootup_oddness(void) 991 { 992 if (rcu_normal) 993 pr_info("\tNo expedited grace period (rcu_normal).\n"); 994 else if (rcu_normal_after_boot) 995 pr_info("\tNo expedited grace period (rcu_normal_after_boot).\n"); 996 else if (rcu_expedited) 997 pr_info("\tAll grace periods are expedited (rcu_expedited).\n"); 998 if (rcu_cpu_stall_suppress) 999 pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n"); 1000 if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT) 1001 pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout); 1002 rcu_tasks_bootup_oddness(); 1003 } 1004 1005 #endif /* #ifndef CONFIG_TINY_RCU */ 1006