fuse: fix blocked_waitq wakeup

BugLink: https://bugs.launchpad.net/bugs/1824553
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
(cherry picked from commit 908a572b80f6e9577b45e81b3dfe2e22111286b8)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index ffcfc0ae3fcfe5c7406c6d9e6f75f406f785b56b..7bb09742b3f9d11a506999c6abe375b1809a0bd0 100644 (file)
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -388,12 +388,19 @@ 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);