The patch below does not apply to the 3.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
>From 908a572b80f6e9577b45e81b3dfe2e22111286b8 Mon Sep 17 00:00:00 2001
From: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Date: Fri, 28 Sep 2018 16:43:22 +0200
Subject: [PATCH] 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@xxxxxxxxxx>
Fixes: 3c18ef8117f0 ("fuse: optimize wake_up")
Cc: <stable@xxxxxxxxxxxxxxx> # v3.10
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);
