+++ /dev/null
-/* rwsem.c: R/W semaphores: contention handling functions
- *
- * Written by David Howells (dhowells@redhat.com).
- * Derived from arch/i386/kernel/semaphore.c
- *
- * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
- * and Michel Lespinasse <walken@google.com>
- */
-#include <linux/rwsem.h>
-#include <linux/sched.h>
-#include <linux/init.h>
-#include <linux/export.h>
-
-/*
- * Initialize an rwsem:
- */
-void __init_rwsem(struct rw_semaphore *sem, const char *name,
- struct lock_class_key *key)
-{
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- /*
- * Make sure we are not reinitializing a held semaphore:
- */
- debug_check_no_locks_freed((void *)sem, sizeof(*sem));
- lockdep_init_map(&sem->dep_map, name, key, 0);
-#endif
- sem->count = RWSEM_UNLOCKED_VALUE;
- raw_spin_lock_init(&sem->wait_lock);
- INIT_LIST_HEAD(&sem->wait_list);
-}
-
-EXPORT_SYMBOL(__init_rwsem);
-
-enum rwsem_waiter_type {
- RWSEM_WAITING_FOR_WRITE,
- RWSEM_WAITING_FOR_READ
-};
-
-struct rwsem_waiter {
- struct list_head list;
- struct task_struct *task;
- enum rwsem_waiter_type type;
-};
-
-enum rwsem_wake_type {
- RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
- RWSEM_WAKE_READERS, /* Wake readers only */
- RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
-};
-
-/*
- * handle the lock release when processes blocked on it that can now run
- * - if we come here from up_xxxx(), then:
- * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
- * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
- * - there must be someone on the queue
- * - the spinlock must be held by the caller
- * - woken process blocks are discarded from the list after having task zeroed
- * - writers are only woken if downgrading is false
- */
-static struct rw_semaphore *
-__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
-{
- struct rwsem_waiter *waiter;
- struct task_struct *tsk;
- struct list_head *next;
- long oldcount, woken, loop, adjustment;
-
- waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
- if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
- if (wake_type == RWSEM_WAKE_ANY)
- /* Wake writer at the front of the queue, but do not
- * grant it the lock yet as we want other writers
- * to be able to steal it. Readers, on the other hand,
- * will block as they will notice the queued writer.
- */
- wake_up_process(waiter->task);
- goto out;
- }
-
- /* Writers might steal the lock before we grant it to the next reader.
- * We prefer to do the first reader grant before counting readers
- * so we can bail out early if a writer stole the lock.
- */
- adjustment = 0;
- if (wake_type != RWSEM_WAKE_READ_OWNED) {
- adjustment = RWSEM_ACTIVE_READ_BIAS;
- try_reader_grant:
- oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
- if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
- /* A writer stole the lock. Undo our reader grant. */
- if (rwsem_atomic_update(-adjustment, sem) &
- RWSEM_ACTIVE_MASK)
- goto out;
- /* Last active locker left. Retry waking readers. */
- goto try_reader_grant;
- }
- }
-
- /* Grant an infinite number of read locks to the readers at the front
- * of the queue. Note we increment the 'active part' of the count by
- * the number of readers before waking any processes up.
- */
- woken = 0;
- do {
- woken++;
-
- if (waiter->list.next == &sem->wait_list)
- break;
-
- waiter = list_entry(waiter->list.next,
- struct rwsem_waiter, list);
-
- } while (waiter->type != RWSEM_WAITING_FOR_WRITE);
-
- adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
- if (waiter->type != RWSEM_WAITING_FOR_WRITE)
- /* hit end of list above */
- adjustment -= RWSEM_WAITING_BIAS;
-
- if (adjustment)
- rwsem_atomic_add(adjustment, sem);
-
- next = sem->wait_list.next;
- loop = woken;
- do {
- waiter = list_entry(next, struct rwsem_waiter, list);
- next = waiter->list.next;
- tsk = waiter->task;
- smp_mb();
- waiter->task = NULL;
- wake_up_process(tsk);
- put_task_struct(tsk);
- } while (--loop);
-
- sem->wait_list.next = next;
- next->prev = &sem->wait_list;
-
- out:
- return sem;
-}
-
-/*
- * wait for the read lock to be granted
- */
-struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
-{
- long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
- struct rwsem_waiter waiter;
- struct task_struct *tsk = current;
-
- /* set up my own style of waitqueue */
- waiter.task = tsk;
- waiter.type = RWSEM_WAITING_FOR_READ;
- get_task_struct(tsk);
-
- raw_spin_lock_irq(&sem->wait_lock);
- if (list_empty(&sem->wait_list))
- adjustment += RWSEM_WAITING_BIAS;
- list_add_tail(&waiter.list, &sem->wait_list);
-
- /* we're now waiting on the lock, but no longer actively locking */
- count = rwsem_atomic_update(adjustment, sem);
-
- /* If there are no active locks, wake the front queued process(es).
- *
- * If there are no writers and we are first in the queue,
- * wake our own waiter to join the existing active readers !
- */
- if (count == RWSEM_WAITING_BIAS ||
- (count > RWSEM_WAITING_BIAS &&
- adjustment != -RWSEM_ACTIVE_READ_BIAS))
- sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
-
- raw_spin_unlock_irq(&sem->wait_lock);
-
- /* wait to be given the lock */
- while (true) {
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- if (!waiter.task)
- break;
- schedule();
- }
-
- tsk->state = TASK_RUNNING;
-
- return sem;
-}
-
-/*
- * wait until we successfully acquire the write lock
- */
-struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
-{
- long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
- struct rwsem_waiter waiter;
- struct task_struct *tsk = current;
-
- /* set up my own style of waitqueue */
- waiter.task = tsk;
- waiter.type = RWSEM_WAITING_FOR_WRITE;
-
- raw_spin_lock_irq(&sem->wait_lock);
- if (list_empty(&sem->wait_list))
- adjustment += RWSEM_WAITING_BIAS;
- list_add_tail(&waiter.list, &sem->wait_list);
-
- /* we're now waiting on the lock, but no longer actively locking */
- count = rwsem_atomic_update(adjustment, sem);
-
- /* If there were already threads queued before us and there are no
- * active writers, the lock must be read owned; so we try to wake
- * any read locks that were queued ahead of us. */
- if (count > RWSEM_WAITING_BIAS &&
- adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
- sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
-
- /* wait until we successfully acquire the lock */
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- while (true) {
- if (!(count & RWSEM_ACTIVE_MASK)) {
- /* Try acquiring the write lock. */
- count = RWSEM_ACTIVE_WRITE_BIAS;
- if (!list_is_singular(&sem->wait_list))
- count += RWSEM_WAITING_BIAS;
-
- if (sem->count == RWSEM_WAITING_BIAS &&
- cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
- RWSEM_WAITING_BIAS)
- break;
- }
-
- raw_spin_unlock_irq(&sem->wait_lock);
-
- /* Block until there are no active lockers. */
- do {
- schedule();
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- } while ((count = sem->count) & RWSEM_ACTIVE_MASK);
-
- raw_spin_lock_irq(&sem->wait_lock);
- }
-
- list_del(&waiter.list);
- raw_spin_unlock_irq(&sem->wait_lock);
- tsk->state = TASK_RUNNING;
-
- return sem;
-}
-
-/*
- * handle waking up a waiter on the semaphore
- * - up_read/up_write has decremented the active part of count if we come here
- */
-struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
-{
- unsigned long flags;
-
- raw_spin_lock_irqsave(&sem->wait_lock, flags);
-
- /* do nothing if list empty */
- if (!list_empty(&sem->wait_list))
- sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
-
- raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
-
- return sem;
-}
-
-/*
- * downgrade a write lock into a read lock
- * - caller incremented waiting part of count and discovered it still negative
- * - just wake up any readers at the front of the queue
- */
-struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
-{
- unsigned long flags;
-
- raw_spin_lock_irqsave(&sem->wait_lock, flags);
-
- /* do nothing if list empty */
- if (!list_empty(&sem->wait_list))
- sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
-
- raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
-
- return sem;
-}
-
-EXPORT_SYMBOL(rwsem_down_read_failed);
-EXPORT_SYMBOL(rwsem_down_write_failed);
-EXPORT_SYMBOL(rwsem_wake);
-EXPORT_SYMBOL(rwsem_downgrade_wake);
projects / linux-2.6-block.git / blobdiff