drivers/tty/tty_ldsem.c

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