fuse: fix blocked_waitq wakeup

Using waitqueue_active() is racy.  Make sure we issue a wake_up()
unconditionally after storing into fc->blocked.  After that it's okay to
optimize with waitqueue_active() since the first wake up provides the
necessary barrier for all waiters, not the just the woken one.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 3c18ef8117f0 ("fuse: optimize wake_up")
Cc: <stable@vger.kernel.org> # v3.10

1 files changed, 11 insertions, 4 deletions

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 34976b42f3e1..51eb602a435b 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -391,12 +391,19 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
         if (test_bit(FR_BACKGROUND, &req->flags)) {
                 spin_lock(&fc->lock);
                 clear_bit(FR_BACKGROUND, &req->flags);
-                if (fc->num_background == fc->max_background)
+                if (fc->num_background == fc->max_background) {
                         fc->blocked = 0;
-
-                /* Wake up next waiter, if any */
-                if (!fc->blocked && waitqueue_active(&fc->blocked_waitq))
                         wake_up(&fc->blocked_waitq);
+                } else if (!fc->blocked) {
+                        /*
+                         * Wake up next waiter, if any.  It's okay to use
+                         * waitqueue_active(), as we've already synced up
+                         * fc->blocked with waiters with the wake_up() call
+                         * above.
+                         */
+                        if (waitqueue_active(&fc->blocked_waitq))
+                                wake_up(&fc->blocked_waitq);
+                }
 
                 if (fc->num_background == fc->congestion_threshold && fc->sb) {
                         clear_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);