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
  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