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-rw-r--r--lib/rwsem.c57
1 files changed, 32 insertions, 25 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 917fd946b495..94f2d7a9dc4f 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);

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