writeback: Fix lock imbalance in writeback_sb_inodes()

Fix bug introduced by 169ebd90.  We have to have wb_list_lock locked when
restarting writeback loop after having waited for inode writeback.

Bug description by Ted Tso:

  I can reproduce this fairly easily by using ext4 w/o a journal, running
  under KVM with 1024megs memory, with fsstress (xfstests #13):

  [   45.153294] =====================================
  [   45.154784] [ BUG: bad unlock balance detected! ]
  [   45.155591] 3.5.0-rc1-00002-gb22b1f1 #124 Not tainted
  [   45.155591] -------------------------------------
  [   45.155591] flush-254:16/2499 is trying to release lock (&(&wb->list_lock)->rlock) at:
  [   45.155591] [<c022c3da>] writeback_sb_inodes+0x160/0x327
  [   45.155591] but there are no more locks to release!

Reported-by: Theodore Ts'o <tytso@mit.edu>
Tested-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 8d2fb8c88cf3..41a3ccff18d8 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -664,6 +664,7 @@ static long writeback_sb_inodes(struct super_block *sb,
                         /* Wait for I_SYNC. This function drops i_lock... */
                         inode_sleep_on_writeback(inode);
                         /* Inode may be gone, start again */
+                        spin_lock(&wb->list_lock);
                         continue;
                 }
                 inode->i_state |= I_SYNC;