[XFS] Prevent deadlock when flushing inodes on unmount

When we are unmounting the filesystem, we flush all the inodes to disk.
Unfortunately, if we have an inode cluster that has just been freed and
marked stale sitting in an incore log buffer (i.e. hasn't been flushed to
disk), it will be holding all the flush locks on the inodes in that
cluster.

xfs_iflush_all() which is called during unmount walks all the inodes
trying to reclaim them, and it doing so calls xfs_finish_reclaim() on each
inode. If the inode is dirty, if grabs the flush lock and flushes it.
Unfortunately, find dirty inodes that already have their flush lock held
and so we sleep.

At this point in the unmount process, we are running single-threaded.
There is nothing more that can push on the log to force the transaction
holding the inode flush locks to disk and hence we deadlock.

The fix is to issue a log force before flushing the inodes on unmount so
that all the flush locks will be released before we start flushing the
inodes.

SGI-PV: 964538
SGI-Modid: xfs-linux-melb:xfs-kern:28862a
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>

fs/xfs/xfs_mount.c

@@ -1182,6 +1182,17 @@ xfs_unmountfs(xfs_mount_t *mp, struct cred *cr)
 	int64_t		fsid;
 #endif
 
+	/*
+	 * We can potentially deadlock here if we have an inode cluster
+	 * that has been freed has it's buffer still pinned in memory because
+	 * the transaction is still sitting in a iclog. The stale inodes
+	 * on that buffer will have their flush locks held until the
+	 * transaction hits the disk and the callbacks run. the inode
+	 * flush takes the flush lock unconditionally and with nothing to
+	 * push out the iclog we will never get that unlocked. hence we
+	 * need to force the log first.
+	 */
+	xfs_log_force(mp, (xfs_lsn_t)0, XFS_LOG_FORCE | XFS_LOG_SYNC);
 	xfs_iflush_all(mp);
 
 	XFS_QM_DQPURGEALL(mp, XFS_QMOPT_QUOTALL | XFS_QMOPT_UMOUNTING);