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 #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         struct task_struct *tsk = current;
 204         long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
 205
 206         /* set up my own style of waitqueue */
 207         raw_spin_lock_irq(&sem->wait_lock);
 208
 209         /* Try to reverse the lock attempt but if the count has changed
 210          * so that reversing fails, check if there are are no waiters,
 211          * and early-out if not */
 212         do {
 213                 if (ldsem_cmpxchg(&count, count + adjust, sem))
 214                         break;
 215                 if (count > 0) {
 216                         raw_spin_unlock_irq(&sem->wait_lock);
 217                         return sem;
 218                 }
 219         } while (1);
 220
 221         list_add_tail(&waiter.list, &sem->read_wait);
 222         sem->wait_readers++;
 223
 224         waiter.task = tsk;
 225         get_task_struct(tsk);
 226
 227         /* if there are no active locks, wake the new lock owner(s) */
 228         if ((count & LDSEM_ACTIVE_MASK) == 0)
 229                 __ldsem_wake(sem);
 230
 231         raw_spin_unlock_irq(&sem->wait_lock);
 232
 233         /* wait to be given the lock */
 234         for (;;) {
 235                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 236
 237                 if (!waiter.task)
 238                         break;
 239                 if (!timeout)
 240                         break;
 241                 timeout = schedule_timeout(timeout);
 242         }
 243
 244         __set_task_state(tsk, TASK_RUNNING);
 245
 246         if (!timeout) {
 247                 /* lock timed out but check if this task was just
 248                  * granted lock ownership - if so, pretend there
 249                  * was no timeout; otherwise, cleanup lock wait */
 250                 raw_spin_lock_irq(&sem->wait_lock);
 251                 if (waiter.task) {
 252                         ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
 253                         list_del(&waiter.list);
 254                         raw_spin_unlock_irq(&sem->wait_lock);
 255                         put_task_struct(waiter.task);
 256                         return NULL;
 257                 }
 258                 raw_spin_unlock_irq(&sem->wait_lock);
 259         }
 260
 261         return sem;
 262 }
 263
 264 /*
 265  * wait for the write lock to be granted
 266  */
 267 static struct ld_semaphore __sched *
 268 down_write_failed(struct ld_semaphore *sem, long count, long timeout)
 269 {
 270         struct ldsem_waiter waiter;
 271         struct task_struct *tsk = current;
 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 = tsk;
 293
 294         set_task_state(tsk, 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_task_state(tsk, 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_task_state(tsk, 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