linux-kernel - [PATCH 3.16 046/305] fuse: fix blocked

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Message-ID: <lsq.1549201508.858517810@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Miklos Szeredi" <mszeredi@...hat.com>
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@...hat.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@...hat.com>
Fixes: 3c18ef8117f0 ("fuse: optimize wake_up")
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.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) {