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 static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem) 90 { 91 long tmp = *old; 92 *old = atomic_long_cmpxchg(&sem->count, *old, new); 93 return *old == tmp; 94 } 95 96 /* 97 * Initialize an ldsem: 98 */ 99 void __init_ldsem(struct ld_semaphore *sem, const char *name, 100 struct lock_class_key *key) 101 { 102 #ifdef CONFIG_DEBUG_LOCK_ALLOC 103 /* 104 * Make sure we are not reinitializing a held semaphore: 105 */ 106 debug_check_no_locks_freed((void *)sem, sizeof(*sem)); 107 lockdep_init_map(&sem->dep_map, name, key, 0); 108 #endif 109 sem->count = LDSEM_UNLOCKED; 110 sem->wait_readers = 0; 111 raw_spin_lock_init(&sem->wait_lock); 112 INIT_LIST_HEAD(&sem->read_wait); 113 INIT_LIST_HEAD(&sem->write_wait); 114 } 115 116 static void __ldsem_wake_readers(struct ld_semaphore *sem) 117 { 118 struct ldsem_waiter *waiter, *next; 119 struct task_struct *tsk; 120 long adjust, count; 121 122 /* Try to grant read locks to all readers on the read wait list. 123 * Note the 'active part' of the count is incremented by 124 * the number of readers before waking any processes up. 125 */ 126 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS); 127 count = ldsem_atomic_update(adjust, sem); 128 do { 129 if (count > 0) 130 break; 131 if (ldsem_cmpxchg(&count, count - adjust, sem)) 132 return; 133 } while (1); 134 135 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { 136 tsk = waiter->task; 137 smp_mb(); 138 waiter->task = NULL; 139 wake_up_process(tsk); 140 put_task_struct(tsk); 141 } 142 INIT_LIST_HEAD(&sem->read_wait); 143 sem->wait_readers = 0; 144 } 145 146 static inline int writer_trylock(struct ld_semaphore *sem) 147 { 148 /* only wake this writer if the active part of the count can be 149 * transitioned from 0 -> 1 150 */ 151 long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem); 152 do { 153 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) 154 return 1; 155 if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem)) 156 return 0; 157 } while (1); 158 } 159 160 static void __ldsem_wake_writer(struct ld_semaphore *sem) 161 { 162 struct ldsem_waiter *waiter; 163 164 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list); 165 wake_up_process(waiter->task); 166 } 167 168 /* 169 * handle the lock release when processes blocked on it that can now run 170 * - if we come here from up_xxxx(), then: 171 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) 172 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) 173 * - the spinlock must be held by the caller 174 * - woken process blocks are discarded from the list after having task zeroed 175 */ 176 static void __ldsem_wake(struct ld_semaphore *sem) 177 { 178 if (!list_empty(&sem->write_wait)) 179 __ldsem_wake_writer(sem); 180 else if (!list_empty(&sem->read_wait)) 181 __ldsem_wake_readers(sem); 182 } 183 184 static void ldsem_wake(struct ld_semaphore *sem) 185 { 186 unsigned long flags; 187 188 raw_spin_lock_irqsave(&sem->wait_lock, flags); 189 __ldsem_wake(sem); 190 raw_spin_unlock_irqrestore(&sem->wait_lock, flags); 191 } 192 193 /* 194 * wait for the read lock to be granted 195 */ 196 static struct ld_semaphore __sched * 197 down_read_failed(struct ld_semaphore *sem, long count, long timeout) 198 { 199 struct ldsem_waiter waiter; 200 struct task_struct *tsk = current; 201 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; 202 203 /* set up my own style of waitqueue */ 204 raw_spin_lock_irq(&sem->wait_lock); 205 206 /* Try to reverse the lock attempt but if the count has changed 207 * so that reversing fails, check if there are are no waiters, 208 * and early-out if not */ 209 do { 210 if (ldsem_cmpxchg(&count, count + adjust, sem)) 211 break; 212 if (count > 0) { 213 raw_spin_unlock_irq(&sem->wait_lock); 214 return sem; 215 } 216 } while (1); 217 218 list_add_tail(&waiter.list, &sem->read_wait); 219 sem->wait_readers++; 220 221 waiter.task = tsk; 222 get_task_struct(tsk); 223 224 /* if there are no active locks, wake the new lock owner(s) */ 225 if ((count & LDSEM_ACTIVE_MASK) == 0) 226 __ldsem_wake(sem); 227 228 raw_spin_unlock_irq(&sem->wait_lock); 229 230 /* wait to be given the lock */ 231 for (;;) { 232 set_task_state(tsk, TASK_UNINTERRUPTIBLE); 233 234 if (!waiter.task) 235 break; 236 if (!timeout) 237 break; 238 timeout = schedule_timeout(timeout); 239 } 240 241 __set_task_state(tsk, TASK_RUNNING); 242 243 if (!timeout) { 244 /* lock timed out but check if this task was just 245 * granted lock ownership - if so, pretend there 246 * was no timeout; otherwise, cleanup lock wait */ 247 raw_spin_lock_irq(&sem->wait_lock); 248 if (waiter.task) { 249 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); 250 list_del(&waiter.list); 251 raw_spin_unlock_irq(&sem->wait_lock); 252 put_task_struct(waiter.task); 253 return NULL; 254 } 255 raw_spin_unlock_irq(&sem->wait_lock); 256 } 257 258 return sem; 259 } 260 261 /* 262 * wait for the write lock to be granted 263 */ 264 static struct ld_semaphore __sched * 265 down_write_failed(struct ld_semaphore *sem, long count, long timeout) 266 { 267 struct ldsem_waiter waiter; 268 struct task_struct *tsk = current; 269 long adjust = -LDSEM_ACTIVE_BIAS; 270 int locked = 0; 271 272 /* set up my own style of waitqueue */ 273 raw_spin_lock_irq(&sem->wait_lock); 274 275 /* Try to reverse the lock attempt but if the count has changed 276 * so that reversing fails, check if the lock is now owned, 277 * and early-out if so */ 278 do { 279 if (ldsem_cmpxchg(&count, count + adjust, sem)) 280 break; 281 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) { 282 raw_spin_unlock_irq(&sem->wait_lock); 283 return sem; 284 } 285 } while (1); 286 287 list_add_tail(&waiter.list, &sem->write_wait); 288 289 waiter.task = tsk; 290 291 set_task_state(tsk, TASK_UNINTERRUPTIBLE); 292 for (;;) { 293 if (!timeout) 294 break; 295 raw_spin_unlock_irq(&sem->wait_lock); 296 timeout = schedule_timeout(timeout); 297 raw_spin_lock_irq(&sem->wait_lock); 298 set_task_state(tsk, TASK_UNINTERRUPTIBLE); 299 if ((locked = writer_trylock(sem))) 300 break; 301 } 302 303 if (!locked) 304 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); 305 list_del(&waiter.list); 306 raw_spin_unlock_irq(&sem->wait_lock); 307 308 __set_task_state(tsk, TASK_RUNNING); 309 310 /* lock wait may have timed out */ 311 if (!locked) 312 return NULL; 313 return sem; 314 } 315 316 317 318 static inline int __ldsem_down_read_nested(struct ld_semaphore *sem, 319 int subclass, long timeout) 320 { 321 long count; 322 323 lockdep_acquire_read(sem, subclass, 0, _RET_IP_); 324 325 count = ldsem_atomic_update(LDSEM_READ_BIAS, sem); 326 if (count <= 0) { 327 lock_stat(sem, contended); 328 if (!down_read_failed(sem, count, timeout)) { 329 lockdep_release(sem, 1, _RET_IP_); 330 return 0; 331 } 332 } 333 lock_stat(sem, acquired); 334 return 1; 335 } 336 337 static inline int __ldsem_down_write_nested(struct ld_semaphore *sem, 338 int subclass, long timeout) 339 { 340 long count; 341 342 lockdep_acquire(sem, subclass, 0, _RET_IP_); 343 344 count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem); 345 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { 346 lock_stat(sem, contended); 347 if (!down_write_failed(sem, count, timeout)) { 348 lockdep_release(sem, 1, _RET_IP_); 349 return 0; 350 } 351 } 352 lock_stat(sem, acquired); 353 return 1; 354 } 355 356 357 /* 358 * lock for reading -- returns 1 if successful, 0 if timed out 359 */ 360 int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout) 361 { 362 might_sleep(); 363 return __ldsem_down_read_nested(sem, 0, timeout); 364 } 365 366 /* 367 * trylock for reading -- returns 1 if successful, 0 if contention 368 */ 369 int ldsem_down_read_trylock(struct ld_semaphore *sem) 370 { 371 long count = sem->count; 372 373 while (count >= 0) { 374 if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) { 375 lockdep_acquire_read(sem, 0, 1, _RET_IP_); 376 lock_stat(sem, acquired); 377 return 1; 378 } 379 } 380 return 0; 381 } 382 383 /* 384 * lock for writing -- returns 1 if successful, 0 if timed out 385 */ 386 int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout) 387 { 388 might_sleep(); 389 return __ldsem_down_write_nested(sem, 0, timeout); 390 } 391 392 /* 393 * trylock for writing -- returns 1 if successful, 0 if contention 394 */ 395 int ldsem_down_write_trylock(struct ld_semaphore *sem) 396 { 397 long count = sem->count; 398 399 while ((count & LDSEM_ACTIVE_MASK) == 0) { 400 if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) { 401 lockdep_acquire(sem, 0, 1, _RET_IP_); 402 lock_stat(sem, acquired); 403 return 1; 404 } 405 } 406 return 0; 407 } 408 409 /* 410 * release a read lock 411 */ 412 void ldsem_up_read(struct ld_semaphore *sem) 413 { 414 long count; 415 416 lockdep_release(sem, 1, _RET_IP_); 417 418 count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem); 419 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0) 420 ldsem_wake(sem); 421 } 422 423 /* 424 * release a write lock 425 */ 426 void ldsem_up_write(struct ld_semaphore *sem) 427 { 428 long count; 429 430 lockdep_release(sem, 1, _RET_IP_); 431 432 count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem); 433 if (count < 0) 434 ldsem_wake(sem); 435 } 436 437 438 #ifdef CONFIG_DEBUG_LOCK_ALLOC 439 440 int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) 441 { 442 might_sleep(); 443 return __ldsem_down_read_nested(sem, subclass, timeout); 444 } 445 446 int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, 447 long timeout) 448 { 449 might_sleep(); 450 return __ldsem_down_write_nested(sem, subclass, timeout); 451 } 452 453 #endif 454