1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Ldisc rw semaphore 4 * 5 * The ldisc semaphore is semantically a rw_semaphore but which enforces 6 * an alternate policy, namely: 7 * 1) Supports lock wait timeouts 8 * 2) Write waiter has priority 9 * 3) Downgrading is not supported 10 * 11 * Implementation notes: 12 * 1) Upper half of semaphore count is a wait count (differs from rwsem 13 * in that rwsem normalizes the upper half to the wait bias) 14 * 2) Lacks overflow checking 15 * 16 * The generic counting was copied and modified from include/asm-generic/rwsem.h 17 * by Paul Mackerras <paulus@samba.org>. 18 * 19 * The scheduling policy was copied and modified from lib/rwsem.c 20 * Written by David Howells (dhowells@redhat.com). 21 * 22 * This implementation incorporates the write lock stealing work of 23 * Michel Lespinasse <walken@google.com>. 24 * 25 * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com> 26 */ 27 28 #include <linux/list.h> 29 #include <linux/spinlock.h> 30 #include <linux/atomic.h> 31 #include <linux/tty.h> 32 #include <linux/sched.h> 33 #include <linux/sched/debug.h> 34 #include <linux/sched/task.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 #define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i) 43 #define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i) 44 #define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i) 45 #define lockdep_release(l, n, i) __rel(l, n, i) 46 #else 47 # define lockdep_acquire(l, s, t, i) do { } while (0) 48 # define lockdep_acquire_nest(l, s, t, n, i) do { } while (0) 49 # define lockdep_acquire_read(l, s, t, i) do { } while (0) 50 # define lockdep_release(l, n, i) do { } while (0) 51 #endif 52 53 #ifdef CONFIG_LOCK_STAT 54 # define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_) 55 #else 56 # define lock_stat(_lock, stat) do { } while (0) 57 #endif 58 59 60 #if BITS_PER_LONG == 64 61 # define LDSEM_ACTIVE_MASK 0xffffffffL 62 #else 63 # define LDSEM_ACTIVE_MASK 0x0000ffffL 64 #endif 65 66 #define LDSEM_UNLOCKED 0L 67 #define LDSEM_ACTIVE_BIAS 1L 68 #define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1) 69 #define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS 70 #define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS) 71 72 struct ldsem_waiter { 73 struct list_head list; 74 struct task_struct *task; 75 }; 76 77 static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem) 78 { 79 return atomic_long_add_return(delta, (atomic_long_t *)&sem->count); 80 } 81 82 /* 83 * ldsem_cmpxchg() updates @*old with the last-known sem->count value. 84 * Returns 1 if count was successfully changed; @*old will have @new value. 85 * Returns 0 if count was not changed; @*old will have most recent sem->count 86 */ 87 static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem) 88 { 89 long tmp = atomic_long_cmpxchg(&sem->count, *old, new); 90 if (tmp == *old) { 91 *old = new; 92 return 1; 93 } else { 94 *old = tmp; 95 return 0; 96 } 97 } 98 99 /* 100 * Initialize an ldsem: 101 */ 102 void __init_ldsem(struct ld_semaphore *sem, const char *name, 103 struct lock_class_key *key) 104 { 105 #ifdef CONFIG_DEBUG_LOCK_ALLOC 106 /* 107 * Make sure we are not reinitializing a held semaphore: 108 */ 109 debug_check_no_locks_freed((void *)sem, sizeof(*sem)); 110 lockdep_init_map(&sem->dep_map, name, key, 0); 111 #endif 112 sem->count = LDSEM_UNLOCKED; 113 sem->wait_readers = 0; 114 raw_spin_lock_init(&sem->wait_lock); 115 INIT_LIST_HEAD(&sem->read_wait); 116 INIT_LIST_HEAD(&sem->write_wait); 117 } 118 119 static void __ldsem_wake_readers(struct ld_semaphore *sem) 120 { 121 struct ldsem_waiter *waiter, *next; 122 struct task_struct *tsk; 123 long adjust, count; 124 125 /* Try to grant read locks to all readers on the read wait list. 126 * Note the 'active part' of the count is incremented by 127 * the number of readers before waking any processes up. 128 */ 129 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS); 130 count = ldsem_atomic_update(adjust, sem); 131 do { 132 if (count > 0) 133 break; 134 if (ldsem_cmpxchg(&count, count - adjust, sem)) 135 return; 136 } while (1); 137 138 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { 139 tsk = waiter->task; 140 smp_mb(); 141 waiter->task = NULL; 142 wake_up_process(tsk); 143 put_task_struct(tsk); 144 } 145 INIT_LIST_HEAD(&sem->read_wait); 146 sem->wait_readers = 0; 147 } 148 149 static inline int writer_trylock(struct ld_semaphore *sem) 150 { 151 /* only wake this writer if the active part of the count can be 152 * transitioned from 0 -> 1 153 */ 154 long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem); 155 do { 156 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) 157 return 1; 158 if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem)) 159 return 0; 160 } while (1); 161 } 162 163 static void __ldsem_wake_writer(struct ld_semaphore *sem) 164 { 165 struct ldsem_waiter *waiter; 166 167 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list); 168 wake_up_process(waiter->task); 169 } 170 171 /* 172 * handle the lock release when processes blocked on it that can now run 173 * - if we come here from up_xxxx(), then: 174 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) 175 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) 176 * - the spinlock must be held by the caller 177 * - woken process blocks are discarded from the list after having task zeroed 178 */ 179 static void __ldsem_wake(struct ld_semaphore *sem) 180 { 181 if (!list_empty(&sem->write_wait)) 182 __ldsem_wake_writer(sem); 183 else if (!list_empty(&sem->read_wait)) 184 __ldsem_wake_readers(sem); 185 } 186 187 static void ldsem_wake(struct ld_semaphore *sem) 188 { 189 unsigned long flags; 190 191 raw_spin_lock_irqsave(&sem->wait_lock, flags); 192 __ldsem_wake(sem); 193 raw_spin_unlock_irqrestore(&sem->wait_lock, flags); 194 } 195 196 /* 197 * wait for the read lock to be granted 198 */ 199 static struct ld_semaphore __sched * 200 down_read_failed(struct ld_semaphore *sem, long count, long timeout) 201 { 202 struct ldsem_waiter waiter; 203 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; 204 205 /* set up my own style of waitqueue */ 206 raw_spin_lock_irq(&sem->wait_lock); 207 208 /* Try to reverse the lock attempt but if the count has changed 209 * so that reversing fails, check if there are are no waiters, 210 * and early-out if not */ 211 do { 212 if (ldsem_cmpxchg(&count, count + adjust, sem)) 213 break; 214 if (count > 0) { 215 raw_spin_unlock_irq(&sem->wait_lock); 216 return sem; 217 } 218 } while (1); 219 220 list_add_tail(&waiter.list, &sem->read_wait); 221 sem->wait_readers++; 222 223 waiter.task = current; 224 get_task_struct(current); 225 226 /* if there are no active locks, wake the new lock owner(s) */ 227 if ((count & LDSEM_ACTIVE_MASK) == 0) 228 __ldsem_wake(sem); 229 230 raw_spin_unlock_irq(&sem->wait_lock); 231 232 /* wait to be given the lock */ 233 for (;;) { 234 set_current_state(TASK_UNINTERRUPTIBLE); 235 236 if (!waiter.task) 237 break; 238 if (!timeout) 239 break; 240 timeout = schedule_timeout(timeout); 241 } 242 243 __set_current_state(TASK_RUNNING); 244 245 if (!timeout) { 246 /* lock timed out but check if this task was just 247 * granted lock ownership - if so, pretend there 248 * was no timeout; otherwise, cleanup lock wait */ 249 raw_spin_lock_irq(&sem->wait_lock); 250 if (waiter.task) { 251 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); 252 list_del(&waiter.list); 253 raw_spin_unlock_irq(&sem->wait_lock); 254 put_task_struct(waiter.task); 255 return NULL; 256 } 257 raw_spin_unlock_irq(&sem->wait_lock); 258 } 259 260 return sem; 261 } 262 263 /* 264 * wait for the write lock to be granted 265 */ 266 static struct ld_semaphore __sched * 267 down_write_failed(struct ld_semaphore *sem, long count, long timeout) 268 { 269 struct ldsem_waiter waiter; 270 long adjust = -LDSEM_ACTIVE_BIAS; 271 int locked = 0; 272 273 /* set up my own style of waitqueue */ 274 raw_spin_lock_irq(&sem->wait_lock); 275 276 /* Try to reverse the lock attempt but if the count has changed 277 * so that reversing fails, check if the lock is now owned, 278 * and early-out if so */ 279 do { 280 if (ldsem_cmpxchg(&count, count + adjust, sem)) 281 break; 282 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) { 283 raw_spin_unlock_irq(&sem->wait_lock); 284 return sem; 285 } 286 } while (1); 287 288 list_add_tail(&waiter.list, &sem->write_wait); 289 290 waiter.task = current; 291 292 set_current_state(TASK_UNINTERRUPTIBLE); 293 for (;;) { 294 if (!timeout) 295 break; 296 raw_spin_unlock_irq(&sem->wait_lock); 297 timeout = schedule_timeout(timeout); 298 raw_spin_lock_irq(&sem->wait_lock); 299 set_current_state(TASK_UNINTERRUPTIBLE); 300 locked = writer_trylock(sem); 301 if (locked) 302 break; 303 } 304 305 if (!locked) 306 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); 307 list_del(&waiter.list); 308 raw_spin_unlock_irq(&sem->wait_lock); 309 310 __set_current_state(TASK_RUNNING); 311 312 /* lock wait may have timed out */ 313 if (!locked) 314 return NULL; 315 return sem; 316 } 317 318 319 320 static int __ldsem_down_read_nested(struct ld_semaphore *sem, 321 int subclass, long timeout) 322 { 323 long count; 324 325 lockdep_acquire_read(sem, subclass, 0, _RET_IP_); 326 327 count = ldsem_atomic_update(LDSEM_READ_BIAS, sem); 328 if (count <= 0) { 329 lock_stat(sem, contended); 330 if (!down_read_failed(sem, count, timeout)) { 331 lockdep_release(sem, 1, _RET_IP_); 332 return 0; 333 } 334 } 335 lock_stat(sem, acquired); 336 return 1; 337 } 338 339 static int __ldsem_down_write_nested(struct ld_semaphore *sem, 340 int subclass, long timeout) 341 { 342 long count; 343 344 lockdep_acquire(sem, subclass, 0, _RET_IP_); 345 346 count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem); 347 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { 348 lock_stat(sem, contended); 349 if (!down_write_failed(sem, count, timeout)) { 350 lockdep_release(sem, 1, _RET_IP_); 351 return 0; 352 } 353 } 354 lock_stat(sem, acquired); 355 return 1; 356 } 357 358 359 /* 360 * lock for reading -- returns 1 if successful, 0 if timed out 361 */ 362 int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout) 363 { 364 might_sleep(); 365 return __ldsem_down_read_nested(sem, 0, timeout); 366 } 367 368 /* 369 * trylock for reading -- returns 1 if successful, 0 if contention 370 */ 371 int ldsem_down_read_trylock(struct ld_semaphore *sem) 372 { 373 long count = sem->count; 374 375 while (count >= 0) { 376 if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) { 377 lockdep_acquire_read(sem, 0, 1, _RET_IP_); 378 lock_stat(sem, acquired); 379 return 1; 380 } 381 } 382 return 0; 383 } 384 385 /* 386 * lock for writing -- returns 1 if successful, 0 if timed out 387 */ 388 int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout) 389 { 390 might_sleep(); 391 return __ldsem_down_write_nested(sem, 0, timeout); 392 } 393 394 /* 395 * trylock for writing -- returns 1 if successful, 0 if contention 396 */ 397 int ldsem_down_write_trylock(struct ld_semaphore *sem) 398 { 399 long count = sem->count; 400 401 while ((count & LDSEM_ACTIVE_MASK) == 0) { 402 if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) { 403 lockdep_acquire(sem, 0, 1, _RET_IP_); 404 lock_stat(sem, acquired); 405 return 1; 406 } 407 } 408 return 0; 409 } 410 411 /* 412 * release a read lock 413 */ 414 void ldsem_up_read(struct ld_semaphore *sem) 415 { 416 long count; 417 418 lockdep_release(sem, 1, _RET_IP_); 419 420 count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem); 421 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0) 422 ldsem_wake(sem); 423 } 424 425 /* 426 * release a write lock 427 */ 428 void ldsem_up_write(struct ld_semaphore *sem) 429 { 430 long count; 431 432 lockdep_release(sem, 1, _RET_IP_); 433 434 count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem); 435 if (count < 0) 436 ldsem_wake(sem); 437 } 438 439 440 #ifdef CONFIG_DEBUG_LOCK_ALLOC 441 442 int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) 443 { 444 might_sleep(); 445 return __ldsem_down_read_nested(sem, subclass, timeout); 446 } 447 448 int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, 449 long timeout) 450 { 451 might_sleep(); 452 return __ldsem_down_write_nested(sem, subclass, timeout); 453 } 454 455 #endif 456