rwsem: simplify rwsem_down_read_failed

When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS
adjustment doesn't cause other readers to block, so we never have to
worry about waking them back after canceling this adjustment in
rwsem_down_read_failed().

We also never want to steal the lock in rwsem_down_read_failed(), so we
don't have to grab the wait_lock either.
Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/rwsem.c

@@ -182,7 +182,6 @@ static int try_get_writer_sem(struct rw_semaphore *sem,
  */
 struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 {
-	enum rwsem_waiter_type type = RWSEM_WAITING_FOR_READ;
 	signed long adjustment = -RWSEM_ACTIVE_READ_BIAS;
 	struct rwsem_waiter waiter;
 	struct task_struct *tsk = current;
@@ -190,7 +189,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 
 	/* set up my own style of waitqueue */
 	waiter.task = tsk;
-	waiter.type = type;
+	waiter.type = RWSEM_WAITING_FOR_READ;
 	get_task_struct(tsk);
 
 	raw_spin_lock_irq(&sem->wait_lock);
@@ -201,17 +200,9 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	/* 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.
-	 *
-	 * 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 there are no active locks, wake the front queued process(es). */
 	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);
 
 	raw_spin_unlock_irq(&sem->wait_lock);
 
@@ -220,15 +211,6 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 		if (!waiter.task)
 			break;
-
-		raw_spin_lock_irq(&sem->wait_lock);
-		/* Try to get the writer sem, may steal from the head writer: */
-		if (type == RWSEM_WAITING_FOR_WRITE)
-			if (try_get_writer_sem(sem, &waiter)) {
-				raw_spin_unlock_irq(&sem->wait_lock);
-				return sem;
-			}
-		raw_spin_unlock_irq(&sem->wait_lock);
 		schedule();
 	}