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@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 908a572b80f6e9577b45e81b3dfe2e22111286b8 Mon Sep 17 00:00:00 2001

From: Miklos Szeredi mszeredi@redhat.com 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@redhat.com Fixes: 3c18ef8117f0 ("fuse: optimize wake_up") Cc: stable@vger.kernel.org # 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);