writeback: fix pin_sb_for_writeback

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@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 10 insertions, 8 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3a066e91ec8d..0609607d3955 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -534,19 +534,21 @@ 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;
 }