1/* 2 * kernel/mutex.c 3 * 4 * Mutexes: blocking mutual exclusion locks 5 * 6 * Started by Ingo Molnar: 7 * 8 * Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> 9 * 10 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and 11 * David Howells for suggestions and improvements. 12 * 13 * - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline 14 * from the -rt tree, where it was originally implemented for rtmutexes 15 * by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale 16 * and Sven Dietrich. 17 * 18 * Also see Documentation/mutex-design.txt. 19 */ 20#include <linux/mutex.h> 21#include <linux/sched.h> 22#include <linux/module.h> 23#include <linux/spinlock.h> 24#include <linux/interrupt.h> 25#include <linux/debug_locks.h> 26 27/* 28 * In the DEBUG case we are using the "NULL fastpath" for mutexes, 29 * which forces all calls into the slowpath: 30 */ 31#ifdef CONFIG_DEBUG_MUTEXES 32# include "mutex-debug.h" 33# include <asm-generic/mutex-null.h> 34#else 35# include "mutex.h" 36# include <asm/mutex.h> 37#endif 38 39void 40__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key) 41{ 42 atomic_set(&lock->count, 1); 43 spin_lock_init(&lock->wait_lock); 44 INIT_LIST_HEAD(&lock->wait_list); 45 mutex_clear_owner(lock); 46 47 debug_mutex_init(lock, name, key); 48} 49 50EXPORT_SYMBOL(__mutex_init); 51 52#ifndef CONFIG_DEBUG_LOCK_ALLOC 53/* 54 * We split the mutex lock/unlock logic into separate fastpath and 55 * slowpath functions, to reduce the register pressure on the fastpath. 56 * We also put the fastpath first in the kernel image, to make sure the 57 * branch is predicted by the CPU as default-untaken. 58 */ 59static __used noinline void __sched 60__mutex_lock_slowpath(atomic_t *lock_count); 61 62/** 63 * mutex_lock - acquire the mutex 64 * @lock: the mutex to be acquired 65 * 66 * Lock the mutex exclusively for this task. If the mutex is not 67 * available right now, it will sleep until it can get it. 68 * 69 * The mutex must later on be released by the same task that 70 * acquired it. Recursive locking is not allowed. The task 71 * may not exit without first unlocking the mutex. Also, kernel 72 * memory where the mutex resides mutex must not be freed with 73 * the mutex still locked. The mutex must first be initialized 74 * (or statically defined) before it can be locked. memset()-ing 75 * the mutex to 0 is not allowed. 76 * 77 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging 78 * checks that will enforce the restrictions and will also do 79 * deadlock debugging. ) 80 * 81 * This function is similar to (but not equivalent to) down(). 82 */ 83void __sched mutex_lock(struct mutex *lock) 84{ 85 might_sleep(); 86 /* 87 * The locking fastpath is the 1->0 transition from 88 * 'unlocked' into 'locked' state. 89 */ 90 __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath); 91 mutex_set_owner(lock); 92} 93 94EXPORT_SYMBOL(mutex_lock); 95#endif 96 97static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count); 98 99/** 100 * mutex_unlock - release the mutex 101 * @lock: the mutex to be released 102 * 103 * Unlock a mutex that has been locked by this task previously. 104 * 105 * This function must not be used in interrupt context. Unlocking 106 * of a not locked mutex is not allowed. 107 * 108 * This function is similar to (but not equivalent to) up(). 109 */ 110void __sched mutex_unlock(struct mutex *lock) 111{ 112 /* 113 * The unlocking fastpath is the 0->1 transition from 'locked' 114 * into 'unlocked' state: 115 */ 116#ifndef CONFIG_DEBUG_MUTEXES 117 /* 118 * When debugging is enabled we must not clear the owner before time, 119 * the slow path will always be taken, and that clears the owner field 120 * after verifying that it was indeed current. 121 */ 122 mutex_clear_owner(lock); 123#endif 124 __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath); 125} 126 127EXPORT_SYMBOL(mutex_unlock); 128 129/* 130 * Lock a mutex (possibly interruptible), slowpath: 131 */ 132static inline int __sched 133__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 134 unsigned long ip) 135{ 136 struct task_struct *task = current; 137 struct mutex_waiter waiter; 138 unsigned long flags; 139 140 preempt_disable(); 141 mutex_acquire(&lock->dep_map, subclass, 0, ip); 142 143#ifdef CONFIG_MUTEX_SPIN_ON_OWNER 144 /* 145 * Optimistic spinning. 146 * 147 * We try to spin for acquisition when we find that there are no 148 * pending waiters and the lock owner is currently running on a 149 * (different) CPU. 150 * 151 * The rationale is that if the lock owner is running, it is likely to 152 * release the lock soon. 153 * 154 * Since this needs the lock owner, and this mutex implementation 155 * doesn't track the owner atomically in the lock field, we need to 156 * track it non-atomically. 157 * 158 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock 159 * to serialize everything. 160 */ 161 162 for (;;) { 163 struct thread_info *owner; 164 165 /* 166 * If we own the BKL, then don't spin. The owner of 167 * the mutex might be waiting on us to release the BKL. 168 */ 169 if (unlikely(current->lock_depth >= 0)) 170 break; 171 172 /* 173 * If there's an owner, wait for it to either 174 * release the lock or go to sleep. 175 */ 176 owner = ACCESS_ONCE(lock->owner); 177 if (owner && !mutex_spin_on_owner(lock, owner)) 178 break; 179 180 if (atomic_cmpxchg(&lock->count, 1, 0) == 1) { 181 lock_acquired(&lock->dep_map, ip); 182 mutex_set_owner(lock); 183 preempt_enable(); 184 return 0; 185 } 186 187 /* 188 * When there's no owner, we might have preempted between the 189 * owner acquiring the lock and setting the owner field. If 190 * we're an RT task that will live-lock because we won't let 191 * the owner complete. 192 */ 193 if (!owner && (need_resched() || rt_task(task))) 194 break; 195 196 /* 197 * The cpu_relax() call is a compiler barrier which forces 198 * everything in this loop to be re-loaded. We don't need 199 * memory barriers as we'll eventually observe the right 200 * values at the cost of a few extra spins. 201 */ 202 cpu_relax(); 203 } 204#endif 205 spin_lock_mutex(&lock->wait_lock, flags); 206 207 debug_mutex_lock_common(lock, &waiter); 208 debug_mutex_add_waiter(lock, &waiter, task_thread_info(task)); 209 210 /* add waiting tasks to the end of the waitqueue (FIFO): */ 211 list_add_tail(&waiter.list, &lock->wait_list); 212 waiter.task = task; 213 214 if (atomic_xchg(&lock->count, -1) == 1) 215 goto done; 216 217 lock_contended(&lock->dep_map, ip); 218 219 for (;;) { 220 /* 221 * Lets try to take the lock again - this is needed even if 222 * we get here for the first time (shortly after failing to 223 * acquire the lock), to make sure that we get a wakeup once 224 * it's unlocked. Later on, if we sleep, this is the 225 * operation that gives us the lock. We xchg it to -1, so 226 * that when we release the lock, we properly wake up the 227 * other waiters: 228 */ 229 if (atomic_xchg(&lock->count, -1) == 1) 230 break; 231 232 /* 233 * got a signal? (This code gets eliminated in the 234 * TASK_UNINTERRUPTIBLE case.) 235 */ 236 if (unlikely(signal_pending_state(state, task))) { 237 mutex_remove_waiter(lock, &waiter, 238 task_thread_info(task)); 239 mutex_release(&lock->dep_map, 1, ip); 240 spin_unlock_mutex(&lock->wait_lock, flags); 241 242 debug_mutex_free_waiter(&waiter); 243 preempt_enable(); 244 return -EINTR; 245 } 246 __set_task_state(task, state); 247 248 /* didnt get the lock, go to sleep: */ 249 spin_unlock_mutex(&lock->wait_lock, flags); 250 preempt_enable_no_resched(); 251 schedule(); 252 preempt_disable(); 253 spin_lock_mutex(&lock->wait_lock, flags); 254 } 255 256done: 257 lock_acquired(&lock->dep_map, ip); 258 /* got the lock - rejoice! */ 259 mutex_remove_waiter(lock, &waiter, current_thread_info()); 260 mutex_set_owner(lock); 261 262 /* set it to 0 if there are no waiters left: */ 263 if (likely(list_empty(&lock->wait_list))) 264 atomic_set(&lock->count, 0); 265 266 spin_unlock_mutex(&lock->wait_lock, flags); 267 268 debug_mutex_free_waiter(&waiter); 269 preempt_enable(); 270 271 return 0; 272} 273 274#ifdef CONFIG_DEBUG_LOCK_ALLOC 275void __sched 276mutex_lock_nested(struct mutex *lock, unsigned int subclass) 277{ 278 might_sleep(); 279 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, _RET_IP_); 280} 281 282EXPORT_SYMBOL_GPL(mutex_lock_nested); 283 284int __sched 285mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass) 286{ 287 might_sleep(); 288 return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_); 289} 290EXPORT_SYMBOL_GPL(mutex_lock_killable_nested); 291 292int __sched 293mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass) 294{ 295 might_sleep(); 296 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 297 subclass, _RET_IP_); 298} 299 300EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested); 301#endif 302 303/* 304 * Release the lock, slowpath: 305 */ 306static inline void 307__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested) 308{ 309 struct mutex *lock = container_of(lock_count, struct mutex, count); 310 unsigned long flags; 311 312 spin_lock_mutex(&lock->wait_lock, flags); 313 mutex_release(&lock->dep_map, nested, _RET_IP_); 314 debug_mutex_unlock(lock); 315 316 /* 317 * some architectures leave the lock unlocked in the fastpath failure 318 * case, others need to leave it locked. In the later case we have to 319 * unlock it here 320 */ 321 if (__mutex_slowpath_needs_to_unlock()) 322 atomic_set(&lock->count, 1); 323 324 if (!list_empty(&lock->wait_list)) { 325 /* get the first entry from the wait-list: */ 326 struct mutex_waiter *waiter = 327 list_entry(lock->wait_list.next, 328 struct mutex_waiter, list); 329 330 debug_mutex_wake_waiter(lock, waiter); 331 332 wake_up_process(waiter->task); 333 } 334 335 spin_unlock_mutex(&lock->wait_lock, flags); 336} 337 338/* 339 * Release the lock, slowpath: 340 */ 341static __used noinline void 342__mutex_unlock_slowpath(atomic_t *lock_count) 343{ 344 __mutex_unlock_common_slowpath(lock_count, 1); 345} 346 347#ifndef CONFIG_DEBUG_LOCK_ALLOC 348/* 349 * Here come the less common (and hence less performance-critical) APIs: 350 * mutex_lock_interruptible() and mutex_trylock(). 351 */ 352static noinline int __sched 353__mutex_lock_killable_slowpath(atomic_t *lock_count); 354 355static noinline int __sched 356__mutex_lock_interruptible_slowpath(atomic_t *lock_count); 357 358/** 359 * mutex_lock_interruptible - acquire the mutex, interruptible 360 * @lock: the mutex to be acquired 361 * 362 * Lock the mutex like mutex_lock(), and return 0 if the mutex has 363 * been acquired or sleep until the mutex becomes available. If a 364 * signal arrives while waiting for the lock then this function 365 * returns -EINTR. 366 * 367 * This function is similar to (but not equivalent to) down_interruptible(). 368 */ 369int __sched mutex_lock_interruptible(struct mutex *lock) 370{ 371 int ret; 372 373 might_sleep(); 374 ret = __mutex_fastpath_lock_retval 375 (&lock->count, __mutex_lock_interruptible_slowpath); 376 if (!ret) 377 mutex_set_owner(lock); 378 379 return ret; 380} 381 382EXPORT_SYMBOL(mutex_lock_interruptible); 383 384int __sched mutex_lock_killable(struct mutex *lock) 385{ 386 int ret; 387 388 might_sleep(); 389 ret = __mutex_fastpath_lock_retval 390 (&lock->count, __mutex_lock_killable_slowpath); 391 if (!ret) 392 mutex_set_owner(lock); 393 394 return ret; 395} 396EXPORT_SYMBOL(mutex_lock_killable); 397 398static __used noinline void __sched 399__mutex_lock_slowpath(atomic_t *lock_count) 400{ 401 struct mutex *lock = container_of(lock_count, struct mutex, count); 402 403 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_); 404} 405 406static noinline int __sched 407__mutex_lock_killable_slowpath(atomic_t *lock_count) 408{ 409 struct mutex *lock = container_of(lock_count, struct mutex, count); 410 411 return __mutex_lock_common(lock, TASK_KILLABLE, 0, _RET_IP_); 412} 413 414static noinline int __sched 415__mutex_lock_interruptible_slowpath(atomic_t *lock_count) 416{ 417 struct mutex *lock = container_of(lock_count, struct mutex, count); 418 419 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, _RET_IP_); 420} 421#endif 422 423/* 424 * Spinlock based trylock, we take the spinlock and check whether we 425 * can get the lock: 426 */ 427static inline int __mutex_trylock_slowpath(atomic_t *lock_count) 428{ 429 struct mutex *lock = container_of(lock_count, struct mutex, count); 430 unsigned long flags; 431 int prev; 432 433 spin_lock_mutex(&lock->wait_lock, flags); 434 435 prev = atomic_xchg(&lock->count, -1); 436 if (likely(prev == 1)) { 437 mutex_set_owner(lock); 438 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_); 439 } 440 441 /* Set it back to 0 if there are no waiters: */ 442 if (likely(list_empty(&lock->wait_list))) 443 atomic_set(&lock->count, 0); 444 445 spin_unlock_mutex(&lock->wait_lock, flags); 446 447 return prev == 1; 448} 449 450/** 451 * mutex_trylock - try to acquire the mutex, without waiting 452 * @lock: the mutex to be acquired 453 * 454 * Try to acquire the mutex atomically. Returns 1 if the mutex 455 * has been acquired successfully, and 0 on contention. 456 * 457 * NOTE: this function follows the spin_trylock() convention, so 458 * it is negated from the down_trylock() return values! Be careful 459 * about this when converting semaphore users to mutexes. 460 * 461 * This function must not be used in interrupt context. The 462 * mutex must be released by the same task that acquired it. 463 */ 464int __sched mutex_trylock(struct mutex *lock) 465{ 466 int ret; 467 468 ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath); 469 if (ret) 470 mutex_set_owner(lock); 471 472 return ret; 473} 474EXPORT_SYMBOL(mutex_trylock); 475 476/** 477 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0 478 * @cnt: the atomic which we are to dec 479 * @lock: the mutex to return holding if we dec to 0 480 * 481 * return true and hold lock if we dec to 0, return false otherwise 482 */ 483int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock) 484{ 485 /* dec if we can't possibly hit 0 */ 486 if (atomic_add_unless(cnt, -1, 1)) 487 return 0; 488 /* we might hit 0, so take the lock */ 489 mutex_lock(lock); 490 if (!atomic_dec_and_test(cnt)) { 491 /* when we actually did the dec, we didn't hit 0 */ 492 mutex_unlock(lock); 493 return 0; 494 } 495 /* we hit 0, and we hold the lock */ 496 return 1; 497} 498EXPORT_SYMBOL(atomic_dec_and_mutex_lock); 499