[PATCH 2/2] writeback: fix __sync_filesystem(sb, 0) on umount

[Surbhi Palande]

From: Dmitry Monakhov <dmonakhov at openvz.org>

BugLink: http://launchpad.net/bugs/543617

__sync_filesystem(sb, 0) is no longer works on umount because sb can
not be pined, Because s_mount sem is downed for write and s_root is NULL.
In fact umount is a special case similar to WB_SYNC_ALL, where
sb is pinned already.

BADCOMMIT: 03ba3782e8dcc5b0e1efe440d33084f066e38cae

Signed-off-by: Dmitry Monakhov <dmonakhov at openvz.org>
(cherry-picked from https://bugzilla.kernel.org/attachment.cgi?id=26224)

Signed-off-by: Surbhi Palande <surbhi.palande at canonical.com>
---
 fs/fs-writeback.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 4102f20..937a8ae 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -583,9 +583,9 @@ static int pin_sb_for_writeback(struct writeback_control *wbc,
 		unpin_sb_for_writeback(psb);
 
 	/*
-	 * Caller must already hold the ref for this
+	 * Caller must already hold the ref for integrity sync, and on umount
 	 */
-	if (wbc->sync_mode == WB_SYNC_ALL) {
+	if (wbc->sync_mode == WB_SYNC_ALL || !sb->s_root) {
 		WARN_ON(!rwsem_is_locked(&sb->s_umount));
 		return 0;
 	}
@@ -597,6 +597,7 @@ static int pin_sb_for_writeback(struct writeback_control *wbc,
 			spin_unlock(&sb_lock);
 			goto pinned;
 		}
+		WARN_ON(1);
 		/*
 		 * umounted, drop rwsem again and fall through to failure
 		 */
-- 
1.7.0.4