lib/rwsem.c

   1 /* rwsem.c: R/W semaphores: contention handling functions
   2  *
   3  * Written by David Howells (dhowells@redhat.com).
   4  * Derived from arch/i386/kernel/semaphore.c
   5  *
   6  * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
   7  * and Michel Lespinasse <walken@google.com>
   8  */
   9 #include <linux/rwsem.h>
  10 #include <linux/sched.h>
  11 #include <linux/init.h>
  12 #include <linux/export.h>
  13
  14 /*
  15  * Initialize an rwsem:
  16  */
  17 void __init_rwsem(struct rw_semaphore *sem, const char *name,
  18                   struct lock_class_key *key)
  19 {
  20 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  21         /*
  22          * Make sure we are not reinitializing a held semaphore:
  23          */
  24         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
  25         lockdep_init_map(&sem->dep_map, name, key, 0);
  26 #endif
  27         sem->count = RWSEM_UNLOCKED_VALUE;
  28         raw_spin_lock_init(&sem->wait_lock);
  29         INIT_LIST_HEAD(&sem->wait_list);
  30 }
  31
  32 EXPORT_SYMBOL(__init_rwsem);
  33
  34 enum rwsem_waiter_type {
  35         RWSEM_WAITING_FOR_WRITE,
  36         RWSEM_WAITING_FOR_READ
  37 };
  38
  39 struct rwsem_waiter {
  40         struct list_head list;
  41         struct task_struct *task;
  42         enum rwsem_waiter_type type;
  43 };
  44
  45 enum rwsem_wake_type {
  46         RWSEM_WAKE_ANY,         /* Wake whatever's at head of wait list */
  47         RWSEM_WAKE_READERS,     /* Wake readers only */
  48         RWSEM_WAKE_READ_OWNED   /* Waker thread holds the read lock */
  49 };
  50
  51 /*
  52  * handle the lock release when processes blocked on it that can now run
  53  * - if we come here from up_xxxx(), then:
  54  *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
  55  *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
  56  * - there must be someone on the queue
  57  * - the spinlock must be held by the caller
  58  * - woken process blocks are discarded from the list after having task zeroed
  59  * - writers are only woken if downgrading is false
  60  */
  61 static struct rw_semaphore *
  62 __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
  63 {
  64         struct rwsem_waiter *waiter;
  65         struct task_struct *tsk;
  66         struct list_head *next;
  67         long oldcount, woken, loop, adjustment;
  68
  69         waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  70         if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
  71                 if (wake_type == RWSEM_WAKE_ANY)
  72                         /* Wake writer at the front of the queue, but do not
  73                          * grant it the lock yet as we want other writers
  74                          * to be able to steal it.  Readers, on the other hand,
  75                          * will block as they will notice the queued writer.
  76                          */
  77                         wake_up_process(waiter->task);
  78                 goto out;
  79         }
  80
  81         /* Writers might steal the lock before we grant it to the next reader.
  82          * We prefer to do the first reader grant before counting readers
  83          * so we can bail out early if a writer stole the lock.
  84          */
  85         adjustment = 0;
  86         if (wake_type != RWSEM_WAKE_READ_OWNED) {
  87                 adjustment = RWSEM_ACTIVE_READ_BIAS;
  88  try_reader_grant:
  89                 oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
  90                 if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
  91                         /* A writer stole the lock. Undo our reader grant. */
  92                         if (rwsem_atomic_update(-adjustment, sem) &
  93                                                 RWSEM_ACTIVE_MASK)
  94                                 goto out;
  95                         /* Last active locker left. Retry waking readers. */
  96                         goto try_reader_grant;
  97                 }
  98         }
  99
 100         /* Grant an infinite number of read locks to the readers at the front
 101          * of the queue.  Note we increment the 'active part' of the count by
 102          * the number of readers before waking any processes up.
 103          */
 104         woken = 0;
 105         do {
 106                 woken++;
 107
 108                 if (waiter->list.next == &sem->wait_list)
 109                         break;
 110
 111                 waiter = list_entry(waiter->list.next,
 112                                         struct rwsem_waiter, list);
 113
 114         } while (waiter->type != RWSEM_WAITING_FOR_WRITE);
 115
 116         adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
 117         if (waiter->type != RWSEM_WAITING_FOR_WRITE)
 118                 /* hit end of list above */
 119                 adjustment -= RWSEM_WAITING_BIAS;
 120
 121         if (adjustment)
 122                 rwsem_atomic_add(adjustment, sem);
 123
 124         next = sem->wait_list.next;
 125         loop = woken;
 126         do {
 127                 waiter = list_entry(next, struct rwsem_waiter, list);
 128                 next = waiter->list.next;
 129                 tsk = waiter->task;
 130                 smp_mb();
 131                 waiter->task = NULL;
 132                 wake_up_process(tsk);
 133                 put_task_struct(tsk);
 134         } while (--loop);
 135
 136         sem->wait_list.next = next;
 137         next->prev = &sem->wait_list;
 138
 139  out:
 140         return sem;
 141 }
 142
 143 /*
 144  * wait for the read lock to be granted
 145  */
 146 struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 147 {
 148         long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
 149         struct rwsem_waiter waiter;
 150         struct task_struct *tsk = current;
 151
 152         /* set up my own style of waitqueue */
 153         waiter.task = tsk;
 154         waiter.type = RWSEM_WAITING_FOR_READ;
 155         get_task_struct(tsk);
 156
 157         raw_spin_lock_irq(&sem->wait_lock);
 158         if (list_empty(&sem->wait_list))
 159                 adjustment += RWSEM_WAITING_BIAS;
 160         list_add_tail(&waiter.list, &sem->wait_list);
 161
 162         /* we're now waiting on the lock, but no longer actively locking */
 163         count = rwsem_atomic_update(adjustment, sem);
 164
 165         /* If there are no active locks, wake the front queued process(es).
 166          *
 167          * If there are no writers and we are first in the queue,
 168          * wake our own waiter to join the existing active readers !
 169          */
 170         if (count == RWSEM_WAITING_BIAS ||
 171             (count > RWSEM_WAITING_BIAS &&
 172              adjustment != -RWSEM_ACTIVE_READ_BIAS))
 173                 sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
 174
 175         raw_spin_unlock_irq(&sem->wait_lock);
 176
 177         /* wait to be given the lock */
 178         while (true) {
 179                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 180                 if (!waiter.task)
 181                         break;
 182                 schedule();
 183         }
 184
 185         tsk->state = TASK_RUNNING;
 186
 187         return sem;
 188 }
 189
 190 /*
 191  * wait until we successfully acquire the write lock
 192  */
 193 struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
 194 {
 195         long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
 196         struct rwsem_waiter waiter;
 197         struct task_struct *tsk = current;
 198
 199         /* set up my own style of waitqueue */
 200         waiter.task = tsk;
 201         waiter.type = RWSEM_WAITING_FOR_WRITE;
 202
 203         raw_spin_lock_irq(&sem->wait_lock);
 204         if (list_empty(&sem->wait_list))
 205                 adjustment += RWSEM_WAITING_BIAS;
 206         list_add_tail(&waiter.list, &sem->wait_list);
 207
 208         /* we're now waiting on the lock, but no longer actively locking */
 209         count = rwsem_atomic_update(adjustment, sem);
 210
 211         /* If there were already threads queued before us and there are no
 212          * active writers, the lock must be read owned; so we try to wake
 213          * any read locks that were queued ahead of us. */
 214         if (count > RWSEM_WAITING_BIAS &&
 215             adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
 216                 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
 217
 218         /* wait until we successfully acquire the lock */
 219         set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 220         while (true) {
 221                 if (!(count & RWSEM_ACTIVE_MASK)) {
 222                         /* Try acquiring the write lock. */
 223                         count = RWSEM_ACTIVE_WRITE_BIAS;
 224                         if (!list_is_singular(&sem->wait_list))
 225                                 count += RWSEM_WAITING_BIAS;
 226
 227                         if (sem->count == RWSEM_WAITING_BIAS &&
 228                             cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
 229                                                         RWSEM_WAITING_BIAS)
 230                                 break;
 231                 }
 232
 233                 raw_spin_unlock_irq(&sem->wait_lock);
 234
 235                 /* Block until there are no active lockers. */
 236                 do {
 237                         schedule();
 238                         set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 239                 } while ((count = sem->count) & RWSEM_ACTIVE_MASK);
 240
 241                 raw_spin_lock_irq(&sem->wait_lock);
 242         }
 243
 244         list_del(&waiter.list);
 245         raw_spin_unlock_irq(&sem->wait_lock);
 246         tsk->state = TASK_RUNNING;
 247
 248         return sem;
 249 }
 250
 251 /*
 252  * handle waking up a waiter on the semaphore
 253  * - up_read/up_write has decremented the active part of count if we come here
 254  */
 255 struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 256 {
 257         unsigned long flags;
 258
 259         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 260
 261         /* do nothing if list empty */
 262         if (!list_empty(&sem->wait_list))
 263                 sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
 264
 265         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 266
 267         return sem;
 268 }
 269
 270 /*
 271  * downgrade a write lock into a read lock
 272  * - caller incremented waiting part of count and discovered it still negative
 273  * - just wake up any readers at the front of the queue
 274  */
 275 struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
 276 {
 277         unsigned long flags;
 278
 279         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 280
 281         /* do nothing if list empty */
 282         if (!list_empty(&sem->wait_list))
 283                 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 284
 285         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 286
 287         return sem;
 288 }
 289
 290 EXPORT_SYMBOL(rwsem_down_read_failed);
 291 EXPORT_SYMBOL(rwsem_down_write_failed);
 292 EXPORT_SYMBOL(rwsem_wake);
 293 EXPORT_SYMBOL(rwsem_downgrade_wake);