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Subject[PATCH 3.16 046/305] fuse: fix blocked_waitq wakeup
3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <mszeredi@redhat.com>

commit 908a572b80f6e9577b45e81b3dfe2e22111286b8 upstream.

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")
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/fuse/dev.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -376,12 +376,19 @@ __releases(fc->lock)
if (req->background) {
req->background = 0;

- 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->connected && fc->bdi_initialized) {
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