[PATCH 12/16] writeback: fix pin_sb_for_writeback

[Stefan Bader]

From: Christoph Hellwig <hch at lst.de>

BugLink: http://bugs.launchpad.net/bugs/585092

commit 29cb48594b873f6193d6327097e504bd3e2314de upstream

We need to check for s_instances to make sure we don't bother working
against a filesystem that is beeing unmounted, and we need to call
put_super to make sure a superblock is freed when we race against
umount.  Also no need to keep sb_lock after we got a reference on it.

Signed-off-by: Christoph Hellwig <hch at lst.de>
Signed-off-by: Jens Axboe <jaxboe at fusionio.com>
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 fs/fs-writeback.c |   18 ++++++++++--------
 1 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 58a8de2..51372bc 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -522,19 +522,21 @@ select_queue:
 static bool pin_sb_for_writeback(struct super_block *sb)
 {
 	spin_lock(&sb_lock);
+	if (list_empty(&sb->s_instances)) {
+		spin_unlock(&sb_lock);
+		return false;
+	}
+
 	sb->s_count++;
+	spin_unlock(&sb_lock);
+
 	if (down_read_trylock(&sb->s_umount)) {
-		if (sb->s_root) {
-			spin_unlock(&sb_lock);
+		if (sb->s_root)
 			return true;
-		}
-		/*
-		 * umounted, drop rwsem again and fall through to failure
-		 */
 		up_read(&sb->s_umount);
 	}
-	sb->s_count--;
-	spin_unlock(&sb_lock);
+
+	put_super(sb);
 	return false;
 }
 
-- 
1.7.0.4