diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/rwsem.c | 61 |
1 files changed, 34 insertions, 27 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c index ceba8e2..917fd94 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -41,7 +41,7 @@ struct rwsem_waiter { * - 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 + * - 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 @@ -54,26 +54,23 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) struct list_head *next; signed long oldcount, woken, loop; + waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); + if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) + goto readers_only; + if (downgrading) - goto dont_wake_writers; + goto out; - /* 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 + /* There's a writer at the front of the queue - try to grant it the + * write lock. However, we only wake this writer if we can transition + * the active part of the count from 0 -> 1 */ - try_again: + try_again_write: oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem) - RWSEM_ACTIVE_BIAS; if (oldcount & RWSEM_ACTIVE_MASK) - goto undo; - - waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); - - /* try to grant a single write lock if there's a writer at the front - * of the queue - note we leave the 'active part' of the count - * incremented by 1 and the waiting part incremented by 0x00010000 - */ - if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) - goto readers_only; + /* Someone grabbed the sem already */ + goto undo_write; /* We must be careful not to touch 'waiter' after we set ->task = NULL. * It is an allocated on the waiter's stack and may become invalid at @@ -87,18 +84,24 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) put_task_struct(tsk); goto out; - /* don't want to wake any writers */ - dont_wake_writers: - waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); - if (waiter->flags & RWSEM_WAITING_FOR_WRITE) - 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) + /* Someone grabbed the sem already */ + goto undo_read; - /* 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 + 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. */ - readers_only: woken = 0; do { woken++; @@ -138,10 +141,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) /* undo the change to the active count, but check for a transition * 1->0 */ - undo: + undo_write: + 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; + goto try_again_read; } /* |