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 #if BITS_PER_LONG == 64 38 # define LDSEM_ACTIVE_MASK 0xffffffffL 39 #else 40 # define LDSEM_ACTIVE_MASK 0x0000ffffL 41 #endif 42 43 #define LDSEM_UNLOCKED 0L 44 #define LDSEM_ACTIVE_BIAS 1L 45 #define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1) 46 #define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS 47 #define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS) 48 49 struct ldsem_waiter { 50 struct list_head list; 51 struct task_struct *task; 52 }; 53 54 /* 55 * Initialize an ldsem: 56 */ 57 void __init_ldsem(struct ld_semaphore *sem, const char *name, 58 struct lock_class_key *key) 59 { 60 #ifdef CONFIG_DEBUG_LOCK_ALLOC 61 /* 62 * Make sure we are not reinitializing a held semaphore: 63 */ 64 debug_check_no_locks_freed((void *)sem, sizeof(*sem)); 65 lockdep_init_map(&sem->dep_map, name, key, 0); 66 #endif 67 atomic_long_set(&sem->count, LDSEM_UNLOCKED); 68 sem->wait_readers = 0; 69 raw_spin_lock_init(&sem->wait_lock); 70 INIT_LIST_HEAD(&sem->read_wait); 71 INIT_LIST_HEAD(&sem->write_wait); 72 } 73 74 static void __ldsem_wake_readers(struct ld_semaphore *sem) 75 { 76 struct ldsem_waiter *waiter, *next; 77 struct task_struct *tsk; 78 long adjust, count; 79 80 /* 81 * Try to grant read locks to all readers on the read wait list. 82 * Note the 'active part' of the count is incremented by 83 * the number of readers before waking any processes up. 84 */ 85 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS); 86 count = atomic_long_add_return(adjust, &sem->count); 87 do { 88 if (count > 0) 89 break; 90 if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust)) 91 return; 92 } while (1); 93 94 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { 95 tsk = waiter->task; 96 smp_mb(); 97 waiter->task = NULL; 98 wake_up_process(tsk); 99 put_task_struct(tsk); 100 } 101 INIT_LIST_HEAD(&sem->read_wait); 102 sem->wait_readers = 0; 103 } 104 105 static inline int writer_trylock(struct ld_semaphore *sem) 106 { 107 /* 108 * Only wake this writer if the active part of the count can be 109 * transitioned from 0 -> 1 110 */ 111 long count = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count); 112 do { 113 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) 114 return 1; 115 if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS)) 116 return 0; 117 } while (1); 118 } 119 120 static void __ldsem_wake_writer(struct ld_semaphore *sem) 121 { 122 struct ldsem_waiter *waiter; 123 124 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list); 125 wake_up_process(waiter->task); 126 } 127 128 /* 129 * handle the lock release when processes blocked on it that can now run 130 * - if we come here from up_xxxx(), then: 131 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) 132 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) 133 * - the spinlock must be held by the caller 134 * - woken process blocks are discarded from the list after having task zeroed 135 */ 136 static void __ldsem_wake(struct ld_semaphore *sem) 137 { 138 if (!list_empty(&sem->write_wait)) 139 __ldsem_wake_writer(sem); 140 else if (!list_empty(&sem->read_wait)) 141 __ldsem_wake_readers(sem); 142 } 143 144 static void ldsem_wake(struct ld_semaphore *sem) 145 { 146 unsigned long flags; 147 148 raw_spin_lock_irqsave(&sem->wait_lock, flags); 149 __ldsem_wake(sem); 150 raw_spin_unlock_irqrestore(&sem->wait_lock, flags); 151 } 152 153 /* 154 * wait for the read lock to be granted 155 */ 156 static struct ld_semaphore __sched * 157 down_read_failed(struct ld_semaphore *sem, long count, long timeout) 158 { 159 struct ldsem_waiter waiter; 160 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; 161 162 /* set up my own style of waitqueue */ 163 raw_spin_lock_irq(&sem->wait_lock); 164 165 /* 166 * Try to reverse the lock attempt but if the count has changed 167 * so that reversing fails, check if there are are no waiters, 168 * and early-out if not 169 */ 170 do { 171 if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) { 172 count += adjust; 173 break; 174 } 175 if (count > 0) { 176 raw_spin_unlock_irq(&sem->wait_lock); 177 return sem; 178 } 179 } while (1); 180 181 list_add_tail(&waiter.list, &sem->read_wait); 182 sem->wait_readers++; 183 184 waiter.task = current; 185 get_task_struct(current); 186 187 /* if there are no active locks, wake the new lock owner(s) */ 188 if ((count & LDSEM_ACTIVE_MASK) == 0) 189 __ldsem_wake(sem); 190 191 raw_spin_unlock_irq(&sem->wait_lock); 192 193 /* wait to be given the lock */ 194 for (;;) { 195 set_current_state(TASK_UNINTERRUPTIBLE); 196 197 if (!waiter.task) 198 break; 199 if (!timeout) 200 break; 201 timeout = schedule_timeout(timeout); 202 } 203 204 __set_current_state(TASK_RUNNING); 205 206 if (!timeout) { 207 /* 208 * Lock timed out but check if this task was just 209 * granted lock ownership - if so, pretend there 210 * was no timeout; otherwise, cleanup lock wait. 211 */ 212 raw_spin_lock_irq(&sem->wait_lock); 213 if (waiter.task) { 214 atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count); 215 sem->wait_readers--; 216 list_del(&waiter.list); 217 raw_spin_unlock_irq(&sem->wait_lock); 218 put_task_struct(waiter.task); 219 return NULL; 220 } 221 raw_spin_unlock_irq(&sem->wait_lock); 222 } 223 224 return sem; 225 } 226 227 /* 228 * wait for the write lock to be granted 229 */ 230 static struct ld_semaphore __sched * 231 down_write_failed(struct ld_semaphore *sem, long count, long timeout) 232 { 233 struct ldsem_waiter waiter; 234 long adjust = -LDSEM_ACTIVE_BIAS; 235 int locked = 0; 236 237 /* set up my own style of waitqueue */ 238 raw_spin_lock_irq(&sem->wait_lock); 239 240 /* 241 * Try to reverse the lock attempt but if the count has changed 242 * so that reversing fails, check if the lock is now owned, 243 * and early-out if so. 244 */ 245 do { 246 if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) 247 break; 248 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) { 249 raw_spin_unlock_irq(&sem->wait_lock); 250 return sem; 251 } 252 } while (1); 253 254 list_add_tail(&waiter.list, &sem->write_wait); 255 256 waiter.task = current; 257 258 set_current_state(TASK_UNINTERRUPTIBLE); 259 for (;;) { 260 if (!timeout) 261 break; 262 raw_spin_unlock_irq(&sem->wait_lock); 263 timeout = schedule_timeout(timeout); 264 raw_spin_lock_irq(&sem->wait_lock); 265 set_current_state(TASK_UNINTERRUPTIBLE); 266 locked = writer_trylock(sem); 267 if (locked) 268 break; 269 } 270 271 if (!locked) 272 atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count); 273 list_del(&waiter.list); 274 275 /* 276 * In case of timeout, wake up every reader who gave the right of way 277 * to writer. Prevent separation readers into two groups: 278 * one that helds semaphore and another that sleeps. 279 * (in case of no contention with a writer) 280 */ 281 if (!locked && list_empty(&sem->write_wait)) 282 __ldsem_wake_readers(sem); 283 284 raw_spin_unlock_irq(&sem->wait_lock); 285 286 __set_current_state(TASK_RUNNING); 287 288 /* lock wait may have timed out */ 289 if (!locked) 290 return NULL; 291 return sem; 292 } 293 294 295 296 static int __ldsem_down_read_nested(struct ld_semaphore *sem, 297 int subclass, long timeout) 298 { 299 long count; 300 301 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_); 302 303 count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count); 304 if (count <= 0) { 305 lock_contended(&sem->dep_map, _RET_IP_); 306 if (!down_read_failed(sem, count, timeout)) { 307 rwsem_release(&sem->dep_map, 1, _RET_IP_); 308 return 0; 309 } 310 } 311 lock_acquired(&sem->dep_map, _RET_IP_); 312 return 1; 313 } 314 315 static int __ldsem_down_write_nested(struct ld_semaphore *sem, 316 int subclass, long timeout) 317 { 318 long count; 319 320 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_); 321 322 count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count); 323 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { 324 lock_contended(&sem->dep_map, _RET_IP_); 325 if (!down_write_failed(sem, count, timeout)) { 326 rwsem_release(&sem->dep_map, 1, _RET_IP_); 327 return 0; 328 } 329 } 330 lock_acquired(&sem->dep_map, _RET_IP_); 331 return 1; 332 } 333 334 335 /* 336 * lock for reading -- returns 1 if successful, 0 if timed out 337 */ 338 int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout) 339 { 340 might_sleep(); 341 return __ldsem_down_read_nested(sem, 0, timeout); 342 } 343 344 /* 345 * trylock for reading -- returns 1 if successful, 0 if contention 346 */ 347 int ldsem_down_read_trylock(struct ld_semaphore *sem) 348 { 349 long count = atomic_long_read(&sem->count); 350 351 while (count >= 0) { 352 if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) { 353 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_); 354 lock_acquired(&sem->dep_map, _RET_IP_); 355 return 1; 356 } 357 } 358 return 0; 359 } 360 361 /* 362 * lock for writing -- returns 1 if successful, 0 if timed out 363 */ 364 int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout) 365 { 366 might_sleep(); 367 return __ldsem_down_write_nested(sem, 0, timeout); 368 } 369 370 /* 371 * trylock for writing -- returns 1 if successful, 0 if contention 372 */ 373 int ldsem_down_write_trylock(struct ld_semaphore *sem) 374 { 375 long count = atomic_long_read(&sem->count); 376 377 while ((count & LDSEM_ACTIVE_MASK) == 0) { 378 if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_WRITE_BIAS)) { 379 rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_); 380 lock_acquired(&sem->dep_map, _RET_IP_); 381 return 1; 382 } 383 } 384 return 0; 385 } 386 387 /* 388 * release a read lock 389 */ 390 void ldsem_up_read(struct ld_semaphore *sem) 391 { 392 long count; 393 394 rwsem_release(&sem->dep_map, 1, _RET_IP_); 395 396 count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count); 397 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0) 398 ldsem_wake(sem); 399 } 400 401 /* 402 * release a write lock 403 */ 404 void ldsem_up_write(struct ld_semaphore *sem) 405 { 406 long count; 407 408 rwsem_release(&sem->dep_map, 1, _RET_IP_); 409 410 count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count); 411 if (count < 0) 412 ldsem_wake(sem); 413 } 414 415 416 #ifdef CONFIG_DEBUG_LOCK_ALLOC 417 418 int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) 419 { 420 might_sleep(); 421 return __ldsem_down_read_nested(sem, subclass, timeout); 422 } 423 424 int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, 425 long timeout) 426 { 427 might_sleep(); 428 return __ldsem_down_write_nested(sem, subclass, timeout); 429 } 430 431 #endif 432