1 files changed, 88 insertions, 62 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index ceba8e28807..f236d7cd5cf 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -36,45 +36,56 @@ 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:
  *   - 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
  */
-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;
 	struct list_head *next;
-	signed long oldcount, woken, loop;
-
-	if (downgrading)
-		goto dont_wake_writers;
-
-	/* 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:
-	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
-						- RWSEM_ACTIVE_BIAS;
-	if (oldcount & RWSEM_ACTIVE_MASK)
-		goto undo;
+	signed long oldcount, woken, loop, adjustment;
 
 	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;
 
+	if (wake_type == RWSEM_WAKE_READ_OWNED)
+		/* Another active reader was observed, so wakeup is not
+		 * likely to succeed. Save the atomic op.
+		 */
+		goto out;
+
+	/* 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
+	 */
+	adjustment = RWSEM_ACTIVE_WRITE_BIAS;
+	if (waiter->list.next == &sem->wait_list)
+		adjustment -= RWSEM_WAITING_BIAS;
+
+ try_again_write:
+	oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
+	if (oldcount & RWSEM_ACTIVE_MASK)
+		/* 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
 	 * any time after that point (due to a wakeup from another source).
@@ -87,18 +98,30 @@ __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)
+ readers_only:
+	/* 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_WAITING_BIAS)
+		/* Someone grabbed the sem for write already */
 		goto out;
 
-	/* 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
+	/* 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++;
@@ -111,16 +134,15 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
 
 	} while (waiter->flags & RWSEM_WAITING_FOR_READ);
 
-	loop = woken;
-	woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
-	if (!downgrading)
-		/* we'd already done one increment earlier */
-		woken -= RWSEM_ACTIVE_BIAS;
+	adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
+	if (waiter->flags & RWSEM_WAITING_FOR_READ)
+		/* hit end of list above */
+		adjustment -= RWSEM_WAITING_BIAS;
 
-	rwsem_atomic_add(woken, sem);
+	rwsem_atomic_add(adjustment, sem);
 
 	next = sem->wait_list.next;
-	for (; loop > 0; loop--) {
+	for (loop = woken; loop > 0; loop--) {
 		waiter = list_entry(next, struct rwsem_waiter, list);
 		next = waiter->list.next;
 		tsk = waiter->task;
@@ -138,10 +160,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
 
 	/* undo the change to the active count, but check for a transition
 	 * 1->0 */
- undo:
-	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
+ undo_write:
+	if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
 		goto out;
-	goto try_again;
+	goto try_again_write;
 }
 
 /*
@@ -149,8 +171,9 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
  */
 static struct rw_semaphore __sched *
 rwsem_down_failed_common(struct rw_semaphore *sem,
-			struct rwsem_waiter *waiter, signed long adjustment)
+			 unsigned int flags, signed long adjustment)
 {
+	struct rwsem_waiter waiter;
 	struct task_struct *tsk = current;
 	signed long count;
 
@@ -158,23 +181,34 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 
 	/* set up my own style of waitqueue */
 	spin_lock_irq(&sem->wait_lock);
-	waiter->task = tsk;
+	waiter.task = tsk;
+	waiter.flags = flags;
 	get_task_struct(tsk);
 
-	list_add_tail(&waiter->list, &sem->wait_list);
+	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 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);
+	/* If there are no active locks, wake the front queued process(es) up.
+	 *
+	 * Alternatively, if we're called from a failed down_write(), 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)
+		sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
+	else if (count > RWSEM_WAITING_BIAS &&
+		 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
+		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 
 	spin_unlock_irq(&sem->wait_lock);
 
 	/* wait to be given the lock */
 	for (;;) {
-		if (!waiter->task)
+		if (!waiter.task)
 			break;
 		schedule();
 		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
@@ -191,12 +225,8 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 asmregparm struct rw_semaphore __sched *
 rwsem_down_read_failed(struct rw_semaphore *sem)
 {
-	struct rwsem_waiter waiter;
-
-	waiter.flags = RWSEM_WAITING_FOR_READ;
-	rwsem_down_failed_common(sem, &waiter,
-				RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
-	return sem;
+	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
+					-RWSEM_ACTIVE_READ_BIAS);
 }
 
 /*
@@ -205,12 +235,8 @@ rwsem_down_read_failed(struct rw_semaphore *sem)
 asmregparm struct rw_semaphore __sched *
 rwsem_down_write_failed(struct rw_semaphore *sem)
 {
-	struct rwsem_waiter waiter;
-
-	waiter.flags = RWSEM_WAITING_FOR_WRITE;
-	rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
-
-	return sem;
+	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
+					-RWSEM_ACTIVE_WRITE_BIAS);
 }
 
 /*
@@ -225,7 +251,7 @@ 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);
 
@@ -245,7 +271,7 @@ 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);