drivers/tty/tty_ldsem.c

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