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