linux/lib/rwsem.c
<<
>>
Prefs
   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 */
  16void __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
  31EXPORT_SYMBOL(__init_rwsem);
  32
  33struct rwsem_waiter {
  34        struct list_head list;
  35        struct task_struct *task;
  36        unsigned int flags;
  37#define RWSEM_WAITING_FOR_READ  0x00000001
  38#define RWSEM_WAITING_FOR_WRITE 0x00000002
  39};
  40
  41/* Wake types for __rwsem_do_wake().  Note that RWSEM_WAKE_NO_ACTIVE and
  42 * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
  43 * since the rwsem value was observed.
  44 */
  45#define RWSEM_WAKE_ANY        0 /* Wake whatever's at head of wait list */
  46#define RWSEM_WAKE_NO_ACTIVE  1 /* rwsem was observed with no active thread */
  47#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */
  48
  49/*
  50 * handle the lock release when processes blocked on it that can now run
  51 * - if we come here from up_xxxx(), then:
  52 *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
  53 *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
  54 * - there must be someone on the queue
  55 * - the spinlock must be held by the caller
  56 * - woken process blocks are discarded from the list after having task zeroed
  57 * - writers are only woken if downgrading is false
  58 */
  59static struct rw_semaphore *
  60__rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
  61{
  62        struct rwsem_waiter *waiter;
  63        struct task_struct *tsk;
  64        struct list_head *next;
  65        signed long woken, loop, adjustment;
  66
  67        waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  68        if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
  69                goto readers_only;
  70
  71        if (wake_type == RWSEM_WAKE_READ_OWNED)
  72                /* Another active reader was observed, so wakeup is not
  73                 * likely to succeed. Save the atomic op.
  74                 */
  75                goto out;
  76
  77        /* Wake up the writing waiter and let the task grab the sem: */
  78        wake_up_process(waiter->task);
  79        goto out;
  80
  81 readers_only:
  82        /* If we come here from up_xxxx(), another thread might have reached
  83         * rwsem_down_failed_common() before we acquired the spinlock and
  84         * woken up a waiter, making it now active.  We prefer to check for
  85         * this first in order to not spend too much time with the spinlock
  86         * held if we're not going to be able to wake up readers in the end.
  87         *
  88         * Note that we do not need to update the rwsem count: any writer
  89         * trying to acquire rwsem will run rwsem_down_write_failed() due
  90         * to the waiting threads and block trying to acquire the spinlock.
  91         *
  92         * We use a dummy atomic update in order to acquire the cache line
  93         * exclusively since we expect to succeed and run the final rwsem
  94         * count adjustment pretty soon.
  95         */
  96        if (wake_type == RWSEM_WAKE_ANY &&
  97            rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
  98                /* Someone grabbed the sem for write already */
  99                goto out;
 100
 101        /* Grant an infinite number of read locks to the readers at the front
 102         * of the queue.  Note we increment the 'active part' of the count by
 103         * the number of readers before waking any processes up.
 104         */
 105        woken = 0;
 106        do {
 107                woken++;
 108
 109                if (waiter->list.next == &sem->wait_list)
 110                        break;
 111
 112                waiter = list_entry(waiter->list.next,
 113                                        struct rwsem_waiter, list);
 114
 115        } while (waiter->flags & RWSEM_WAITING_FOR_READ);
 116
 117        adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
 118        if (waiter->flags & RWSEM_WAITING_FOR_READ)
 119                /* hit end of list above */
 120                adjustment -= RWSEM_WAITING_BIAS;
 121
 122        rwsem_atomic_add(adjustment, sem);
 123
 124        next = sem->wait_list.next;
 125        for (loop = woken; loop > 0; loop--) {
 126                waiter = list_entry(next, struct rwsem_waiter, list);
 127                next = waiter->list.next;
 128                tsk = waiter->task;
 129                smp_mb();
 130                waiter->task = NULL;
 131                wake_up_process(tsk);
 132                put_task_struct(tsk);
 133        }
 134
 135        sem->wait_list.next = next;
 136        next->prev = &sem->wait_list;
 137
 138 out:
 139        return sem;
 140}
 141
 142/* Try to get write sem, caller holds sem->wait_lock: */
 143static int try_get_writer_sem(struct rw_semaphore *sem,
 144                                        struct rwsem_waiter *waiter)
 145{
 146        struct rwsem_waiter *fwaiter;
 147        long oldcount, adjustment;
 148
 149        /* only steal when first waiter is writing */
 150        fwaiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
 151        if (!(fwaiter->flags & RWSEM_WAITING_FOR_WRITE))
 152                return 0;
 153
 154        adjustment = RWSEM_ACTIVE_WRITE_BIAS;
 155        /* Only one waiter in the queue: */
 156        if (fwaiter == waiter && waiter->list.next == &sem->wait_list)
 157                adjustment -= RWSEM_WAITING_BIAS;
 158
 159try_again_write:
 160        oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
 161        if (!(oldcount & RWSEM_ACTIVE_MASK)) {
 162                /* No active lock: */
 163                struct task_struct *tsk = waiter->task;
 164
 165                list_del(&waiter->list);
 166                smp_mb();
 167                put_task_struct(tsk);
 168                tsk->state = TASK_RUNNING;
 169                return 1;
 170        }
 171        /* some one grabbed the sem already */
 172        if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
 173                return 0;
 174        goto try_again_write;
 175}
 176
 177/*
 178 * wait for a lock to be granted
 179 */
 180static struct rw_semaphore __sched *
 181rwsem_down_failed_common(struct rw_semaphore *sem,
 182                         unsigned int flags, signed long adjustment)
 183{
 184        struct rwsem_waiter waiter;
 185        struct task_struct *tsk = current;
 186        signed long count;
 187
 188        set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 189
 190        /* set up my own style of waitqueue */
 191        raw_spin_lock_irq(&sem->wait_lock);
 192        waiter.task = tsk;
 193        waiter.flags = flags;
 194        get_task_struct(tsk);
 195
 196        if (list_empty(&sem->wait_list))
 197                adjustment += RWSEM_WAITING_BIAS;
 198        list_add_tail(&waiter.list, &sem->wait_list);
 199
 200        /* we're now waiting on the lock, but no longer actively locking */
 201        count = rwsem_atomic_update(adjustment, sem);
 202
 203        /* If there are no active locks, wake the front queued process(es) up.
 204         *
 205         * Alternatively, if we're called from a failed down_write(), there
 206         * were already threads queued before us and there are no active
 207         * writers, the lock must be read owned; so we try to wake any read
 208         * locks that were queued ahead of us. */
 209        if (count == RWSEM_WAITING_BIAS)
 210                sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
 211        else if (count > RWSEM_WAITING_BIAS &&
 212                 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
 213                sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 214
 215        raw_spin_unlock_irq(&sem->wait_lock);
 216
 217        /* wait to be given the lock */
 218        for (;;) {
 219                if (!waiter.task)
 220                        break;
 221
 222                raw_spin_lock_irq(&sem->wait_lock);
 223                /* Try to get the writer sem, may steal from the head writer: */
 224                if (flags == RWSEM_WAITING_FOR_WRITE)
 225                        if (try_get_writer_sem(sem, &waiter)) {
 226                                raw_spin_unlock_irq(&sem->wait_lock);
 227                                return sem;
 228                        }
 229                raw_spin_unlock_irq(&sem->wait_lock);
 230                schedule();
 231                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 232        }
 233
 234        tsk->state = TASK_RUNNING;
 235
 236        return sem;
 237}
 238
 239/*
 240 * wait for the read lock to be granted
 241 */
 242struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 243{
 244        return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
 245                                        -RWSEM_ACTIVE_READ_BIAS);
 246}
 247
 248/*
 249 * wait for the write lock to be granted
 250 */
 251struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
 252{
 253        return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
 254                                        -RWSEM_ACTIVE_WRITE_BIAS);
 255}
 256
 257/*
 258 * handle waking up a waiter on the semaphore
 259 * - up_read/up_write has decremented the active part of count if we come here
 260 */
 261struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 262{
 263        unsigned long flags;
 264
 265        raw_spin_lock_irqsave(&sem->wait_lock, flags);
 266
 267        /* do nothing if list empty */
 268        if (!list_empty(&sem->wait_list))
 269                sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
 270
 271        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 272
 273        return sem;
 274}
 275
 276/*
 277 * downgrade a write lock into a read lock
 278 * - caller incremented waiting part of count and discovered it still negative
 279 * - just wake up any readers at the front of the queue
 280 */
 281struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
 282{
 283        unsigned long flags;
 284
 285        raw_spin_lock_irqsave(&sem->wait_lock, flags);
 286
 287        /* do nothing if list empty */
 288        if (!list_empty(&sem->wait_list))
 289                sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 290
 291        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 292
 293        return sem;
 294}
 295
 296EXPORT_SYMBOL(rwsem_down_read_failed);
 297EXPORT_SYMBOL(rwsem_down_write_failed);
 298EXPORT_SYMBOL(rwsem_wake);
 299EXPORT_SYMBOL(rwsem_downgrade_wake);
 300
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.