From 908a572b80f6e9577b45e81b3dfe2e22111286b8 Mon Sep 17 00:00:00 2001
From: Miklos Szeredi <mszeredi@redhat.com>
Date: Fri Sep 28 16:43:22 2018 +0200
Subject: [PATCH] fuse: fix blocked_waitq wakeup
Git-commit: 908a572b80f6e9577b45e81b3dfe2e22111286b8
References: git-fixes
Patch-mainline: v4.20-rc1

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
Acked-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 34976b4..51eb602 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);