1 /* 2 * Ldisc rw semaphore 3 * 4 * The ldisc semaphore is semantically a rw_semaphore but which enforces 5 * an alternate policy, namely: 6 * 1) Supports lock wait timeouts 7 * 2) Write waiter has priority 8 * 3) Downgrading is not supported 9 * 10 * Implementation notes: 11 * 1) Upper half of semaphore count is a wait count (differs from rwsem 12 * in that rwsem normalizes the upper half to the wait bias) 13 * 2) Lacks overflow checking 14 * 15 * The generic counting was copied and modified from include/asm-generic/rwsem.h 16 * by Paul Mackerras <paulus@samba.org>. 17 * 18 * The scheduling policy was copied and modified from lib/rwsem.c 19 * Written by David Howells (dhowells@redhat.com). 20 * 21 * This implementation incorporates the write lock stealing work of 22 * Michel Lespinasse <walken@google.com>. 23 * 24 * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com> 25 * 26 * This file may be redistributed under the terms of the GNU General Public 27 * License v2. 28 */ 29 30 #include <linux/list.h> 31 #include <linux/spinlock.h> 32 #include <linux/atomic.h> 33 #include <linux/tty.h> 34 #include <linux/sched.h> 35 36 37 #ifdef CONFIG_DEBUG_LOCK_ALLOC 38 # define __acq(l, s, t, r, c, n, i) \ 39 lock_acquire(&(l)->dep_map, s, t, r, c, n, i) 40 # define __rel(l, n, i) \ 41 lock_release(&(l)->dep_map, n, i) 42 # ifdef CONFIG_PROVE_LOCKING 43 # define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 2, NULL, i) 44 # define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 2, n, i) 45 # define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 2, NULL, i) 46 # define lockdep_release(l, n, i) __rel(l, n, i) 47 # else 48 # define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i) 49 # define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i) 50 # define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i) 51 # define lockdep_release(l, n, i) __rel(l, n, i) 52 # endif 53 #else 54 # define lockdep_acquire(l, s, t, i) do { } while (0) 55 # define lockdep_acquire_nest(l, s, t, n, i) do { } while (0) 56 # define lockdep_acquire_read(l, s, t, i) do { } while (0) 57 # define lockdep_release(l, n, i) do { } while (0) 58 #endif 59 60 #ifdef CONFIG_LOCK_STAT 61 # define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_) 62 #else 63 # define lock_stat(_lock, stat) do { } while (0) 64 #endif 65 66 67 #if BITS_PER_LONG == 64 68 # define LDSEM_ACTIVE_MASK 0xffffffffL 69 #else 70 # define LDSEM_ACTIVE_MASK 0x0000ffffL 71 #endif 72 73 #define LDSEM_UNLOCKED 0L 74 #define LDSEM_ACTIVE_BIAS 1L 75 #define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1) 76 #define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS 77 #define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS) 78 79 struct ldsem_waiter { 80 struct list_head list; 81 struct task_struct *task; 82 }; 83 84 static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem) 85 { 86 return atomic_long_add_return(delta, (atomic_long_t *)&sem->count); 87 } 88 89 /* 90 * ldsem_cmpxchg() updates @*old with the last-known sem->count value. 91 * Returns 1 if count was successfully changed; @*old will have @new value. 92 * Returns 0 if count was not changed; @*old will have most recent sem->count 93 */ 94 static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem) 95 { 96 long tmp = atomic_long_cmpxchg(&sem->count, *old, new); 97 if (tmp == *old) { 98 *old = new; 99 return 1; 100 } else { 101 *old = tmp; 102 return 0; 103 } 104 } 105 106 /* 107 * Initialize an ldsem: 108 */ 109 void __init_ldsem(struct ld_semaphore *sem, const char *name, 110 struct lock_class_key *key) 111 { 112 #ifdef CONFIG_DEBUG_LOCK_ALLOC 113 /* 114 * Make sure we are not reinitializing a held semaphore: 115 */ 116 debug_check_no_locks_freed((void *)sem, sizeof(*sem)); 117 lockdep_init_map(&sem->dep_map, name, key, 0); 118 #endif 119 sem->count = LDSEM_UNLOCKED; 120 sem->wait_readers = 0; 121 raw_spin_lock_init(&sem->wait_lock); 122 INIT_LIST_HEAD(&sem->read_wait); 123 INIT_LIST_HEAD(&sem->write_wait); 124 } 125 126 static void __ldsem_wake_readers(struct ld_semaphore *sem) 127 { 128 struct ldsem_waiter *waiter, *next; 129 struct task_struct *tsk; 130 long adjust, count; 131 132 /* Try to grant read locks to all readers on the read wait list. 133 * Note the 'active part' of the count is incremented by 134 * the number of readers before waking any processes up. 135 */ 136 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS); 137 count = ldsem_atomic_update(adjust, sem); 138 do { 139 if (count > 0) 140 break; 141 if (ldsem_cmpxchg(&count, count - adjust, sem)) 142 return; 143 } while (1); 144 145 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { 146 tsk = waiter->task; 147 smp_mb(); 148 waiter->task = NULL; 149 wake_up_process(tsk); 150 put_task_struct(tsk); 151 } 152 INIT_LIST_HEAD(&sem->read_wait); 153 sem->wait_readers = 0; 154 } 155 156 static inline int writer_trylock(struct ld_semaphore *sem) 157 { 158 /* only wake this writer if the active part of the count can be 159 * transitioned from 0 -> 1 160 */ 161 long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem); 162 do { 163 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) 164 return 1; 165 if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem)) 166 return 0; 167 } while (1); 168 } 169 170 static void __ldsem_wake_writer(struct ld_semaphore *sem) 171 { 172 struct ldsem_waiter *waiter; 173 174 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list); 175 wake_up_process(waiter->task); 176 } 177 178 /* 179 * handle the lock release when processes blocked on it that can now run 180 * - if we come here from up_xxxx(), then: 181 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) 182 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) 183 * - the spinlock must be held by the caller 184 * - woken process blocks are discarded from the list after having task zeroed 185 */ 186 static void __ldsem_wake(struct ld_semaphore *sem) 187 { 188 if (!list_empty(&sem->write_wait)) 189 __ldsem_wake_writer(sem); 190 else if (!list_empty(&sem->read_wait)) 191 __ldsem_wake_readers(sem); 192 } 193 194 static void ldsem_wake(struct ld_semaphore *sem) 195 { 196 unsigned long flags; 197 198 raw_spin_lock_irqsave(&sem->wait_lock, flags); 199 __ldsem_wake(sem); 200 raw_spin_unlock_irqrestore(&sem->wait_lock, flags); 201 } 202 203 /* 204 * wait for the read lock to be granted 205 */ 206 static struct ld_semaphore __sched * 207 down_read_failed(struct ld_semaphore *sem, long count, long timeout) 208 { 209 struct ldsem_waiter waiter; 210 struct task_struct *tsk = current; 211 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; 212 213 /* set up my own style of waitqueue */ 214 raw_spin_lock_irq(&sem->wait_lock); 215 216 /* Try to reverse the lock attempt but if the count has changed 217 * so that reversing fails, check if there are are no waiters, 218 * and early-out if not */ 219 do { 220 if (ldsem_cmpxchg(&count, count + adjust, sem)) 221 break; 222 if (count > 0) { 223 raw_spin_unlock_irq(&sem->wait_lock); 224 return sem; 225 } 226 } while (1); 227 228 list_add_tail(&waiter.list, &sem->read_wait); 229 sem->wait_readers++; 230 231 waiter.task = tsk; 232 get_task_struct(tsk); 233 234 /* if there are no active locks, wake the new lock owner(s) */ 235 if ((count & LDSEM_ACTIVE_MASK) == 0) 236 __ldsem_wake(sem); 237 238 raw_spin_unlock_irq(&sem->wait_lock); 239 240 /* wait to be given the lock */ 241 for (;;) { 242 set_task_state(tsk, TASK_UNINTERRUPTIBLE); 243 244 if (!waiter.task) 245 break; 246 if (!timeout) 247 break; 248 timeout = schedule_timeout(timeout); 249 } 250 251 __set_task_state(tsk, TASK_RUNNING); 252 253 if (!timeout) { 254 /* lock timed out but check if this task was just 255 * granted lock ownership - if so, pretend there 256 * was no timeout; otherwise, cleanup lock wait */ 257 raw_spin_lock_irq(&sem->wait_lock); 258 if (waiter.task) { 259 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); 260 list_del(&waiter.list); 261 raw_spin_unlock_irq(&sem->wait_lock); 262 put_task_struct(waiter.task); 263 return NULL; 264 } 265 raw_spin_unlock_irq(&sem->wait_lock); 266 } 267 268 return sem; 269 } 270 271 /* 272 * wait for the write lock to be granted 273 */ 274 static struct ld_semaphore __sched * 275 down_write_failed(struct ld_semaphore *sem, long count, long timeout) 276 { 277 struct ldsem_waiter waiter; 278 struct task_struct *tsk = current; 279 long adjust = -LDSEM_ACTIVE_BIAS; 280 int locked = 0; 281 282 /* set up my own style of waitqueue */ 283 raw_spin_lock_irq(&sem->wait_lock); 284 285 /* Try to reverse the lock attempt but if the count has changed 286 * so that reversing fails, check if the lock is now owned, 287 * and early-out if so */ 288 do { 289 if (ldsem_cmpxchg(&count, count + adjust, sem)) 290 break; 291 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) { 292 raw_spin_unlock_irq(&sem->wait_lock); 293 return sem; 294 } 295 } while (1); 296 297 list_add_tail(&waiter.list, &sem->write_wait); 298 299 waiter.task = tsk; 300 301 set_task_state(tsk, TASK_UNINTERRUPTIBLE); 302 for (;;) { 303 if (!timeout) 304 break; 305 raw_spin_unlock_irq(&sem->wait_lock); 306 timeout = schedule_timeout(timeout); 307 raw_spin_lock_irq(&sem->wait_lock); 308 set_task_state(tsk, TASK_UNINTERRUPTIBLE); 309 if ((locked = writer_trylock(sem))) 310 break; 311 } 312 313 if (!locked) 314 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); 315 list_del(&waiter.list); 316 raw_spin_unlock_irq(&sem->wait_lock); 317 318 __set_task_state(tsk, TASK_RUNNING); 319 320 /* lock wait may have timed out */ 321 if (!locked) 322 return NULL; 323 return sem; 324 } 325 326 327 328 static inline int __ldsem_down_read_nested(struct ld_semaphore *sem, 329 int subclass, long timeout) 330 { 331 long count; 332 333 lockdep_acquire_read(sem, subclass, 0, _RET_IP_); 334 335 count = ldsem_atomic_update(LDSEM_READ_BIAS, sem); 336 if (count <= 0) { 337 lock_stat(sem, contended); 338 if (!down_read_failed(sem, count, timeout)) { 339 lockdep_release(sem, 1, _RET_IP_); 340 return 0; 341 } 342 } 343 lock_stat(sem, acquired); 344 return 1; 345 } 346 347 static inline int __ldsem_down_write_nested(struct ld_semaphore *sem, 348 int subclass, long timeout) 349 { 350 long count; 351 352 lockdep_acquire(sem, subclass, 0, _RET_IP_); 353 354 count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem); 355 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { 356 lock_stat(sem, contended); 357 if (!down_write_failed(sem, count, timeout)) { 358 lockdep_release(sem, 1, _RET_IP_); 359 return 0; 360 } 361 } 362 lock_stat(sem, acquired); 363 return 1; 364 } 365 366 367 /* 368 * lock for reading -- returns 1 if successful, 0 if timed out 369 */ 370 int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout) 371 { 372 might_sleep(); 373 return __ldsem_down_read_nested(sem, 0, timeout); 374 } 375 376 /* 377 * trylock for reading -- returns 1 if successful, 0 if contention 378 */ 379 int ldsem_down_read_trylock(struct ld_semaphore *sem) 380 { 381 long count = sem->count; 382 383 while (count >= 0) { 384 if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) { 385 lockdep_acquire_read(sem, 0, 1, _RET_IP_); 386 lock_stat(sem, acquired); 387 return 1; 388 } 389 } 390 return 0; 391 } 392 393 /* 394 * lock for writing -- returns 1 if successful, 0 if timed out 395 */ 396 int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout) 397 { 398 might_sleep(); 399 return __ldsem_down_write_nested(sem, 0, timeout); 400 } 401 402 /* 403 * trylock for writing -- returns 1 if successful, 0 if contention 404 */ 405 int ldsem_down_write_trylock(struct ld_semaphore *sem) 406 { 407 long count = sem->count; 408 409 while ((count & LDSEM_ACTIVE_MASK) == 0) { 410 if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) { 411 lockdep_acquire(sem, 0, 1, _RET_IP_); 412 lock_stat(sem, acquired); 413 return 1; 414 } 415 } 416 return 0; 417 } 418 419 /* 420 * release a read lock 421 */ 422 void ldsem_up_read(struct ld_semaphore *sem) 423 { 424 long count; 425 426 lockdep_release(sem, 1, _RET_IP_); 427 428 count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem); 429 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0) 430 ldsem_wake(sem); 431 } 432 433 /* 434 * release a write lock 435 */ 436 void ldsem_up_write(struct ld_semaphore *sem) 437 { 438 long count; 439 440 lockdep_release(sem, 1, _RET_IP_); 441 442 count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem); 443 if (count < 0) 444 ldsem_wake(sem); 445 } 446 447 448 #ifdef CONFIG_DEBUG_LOCK_ALLOC 449 450 int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) 451 { 452 might_sleep(); 453 return __ldsem_down_read_nested(sem, subclass, timeout); 454 } 455 456 int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, 457 long timeout) 458 { 459 might_sleep(); 460 return __ldsem_down_write_nested(sem, subclass, timeout); 461 } 462 463 #endif 464