1 files changed, 32 insertions, 25 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 917fd94..94f2d7a 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);