diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/rwsem.c | 57 |
1 files changed, 32 insertions, 25 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c index 917fd946b49..94f2d7a9dc4 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -36,6 +36,14 @@ struct rwsem_waiter { #define RWSEM_WAITING_FOR_WRITE 0x00000002 }; +/* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and + * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held + * since the rwsem value was observed. + */ +#define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ +#define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ +#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ + /* * handle the lock release when processes blocked on it that can now run * - if we come here from up_xxxx(), then: @@ -46,8 +54,8 @@ struct rwsem_waiter { * - woken process blocks are discarded from the list after having task zeroed * - writers are only woken if downgrading is false */ -static inline struct rw_semaphore * -__rwsem_do_wake(struct rw_semaphore *sem, int downgrading) +static struct rw_semaphore * +__rwsem_do_wake(struct rw_semaphore *sem, int wake_type) { struct rwsem_waiter *waiter; struct task_struct *tsk; @@ -58,7 +66,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) goto readers_only; - if (downgrading) + if (wake_type == RWSEM_WAKE_READ_OWNED) goto out; /* There's a writer at the front of the queue - try to grant it the @@ -85,19 +93,25 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) goto out; readers_only: - if (downgrading) - goto wake_readers; - - /* if we came through an up_xxxx() call, we only only wake someone up - * if we can transition the active part of the count from 0 -> 1 */ - try_again_read: - oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem) - - RWSEM_ACTIVE_BIAS; - if (oldcount & RWSEM_ACTIVE_MASK) + /* If we come here from up_xxxx(), another thread might have reached + * rwsem_down_failed_common() before we acquired the spinlock and + * woken up a waiter, making it now active. We prefer to check for + * this first in order to not spend too much time with the spinlock + * held if we're not going to be able to wake up readers in the end. + * + * Note that we do not need to update the rwsem count: any writer + * trying to acquire rwsem will run rwsem_down_write_failed() due + * to the waiting threads and block trying to acquire the spinlock. + * + * We use a dummy atomic update in order to acquire the cache line + * exclusively since we expect to succeed and run the final rwsem + * count adjustment pretty soon. + */ + if (wake_type == RWSEM_WAKE_ANY && + (rwsem_atomic_update(0, sem) & RWSEM_ACTIVE_MASK)) /* Someone grabbed the sem already */ - goto undo_read; + goto out; - wake_readers: /* 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. @@ -116,9 +130,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) loop = woken; woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS; - if (!downgrading) - /* we'd already done one increment earlier */ - woken -= RWSEM_ACTIVE_BIAS; rwsem_atomic_add(woken, sem); @@ -145,10 +156,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK) goto out; goto try_again_write; - undo_read: - if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK) - goto out; - goto try_again_read; } /* @@ -170,12 +177,12 @@ rwsem_down_failed_common(struct rw_semaphore *sem, list_add_tail(&waiter->list, &sem->wait_list); - /* we're now waiting on the lock, but no longer actively read-locking */ + /* 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) up */ if (!(count & RWSEM_ACTIVE_MASK)) - sem = __rwsem_do_wake(sem, 0); + sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); spin_unlock_irq(&sem->wait_lock); @@ -232,7 +239,7 @@ asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) /* do nothing if list empty */ if (!list_empty(&sem->wait_list)) - sem = __rwsem_do_wake(sem, 0); + sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); spin_unlock_irqrestore(&sem->wait_lock, flags); @@ -252,7 +259,7 @@ asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) /* do nothing if list empty */ if (!list_empty(&sem->wait_list)) - sem = __rwsem_do_wake(sem, 1); + sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); spin_unlock_irqrestore(&sem->wait_lock, flags); |