xfs: wait for the write the superblock on unmount

v2: Add the xfs_buf_lock to xfs_quiesce_attr().
    Add explaination why xfs_buf_lock() is used to wait for write.

xfs_wait_buftarg() does not wait for the completion of the write of the
uncached superblock. This write can race with the shutdown of the log
and causes a panic if the write does not win the race.

During the log write, xfsaild_push() will lock the buffer and set the
XBF_ASYNC flag. Because the XBF_FLAG is set, complete() is not performed
on the buffer's iowait entry, we cannot call xfs_buf_iowait() to wait
for the write to complete. The buffer's lock is held until the write is
complete, so we can block on a xfs_buf_lock() request to be notified
that the write is complete.
Signed-off-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_mount.c

@@ -1529,6 +1529,15 @@ xfs_unmountfs(
 	xfs_ail_push_all_sync(mp->m_ail);
 	xfs_wait_buftarg(mp->m_ddev_targp);
 
+	/*
+	 * The superblock buffer is uncached and xfsaild_push() will lock and
+	 * set the XBF_ASYNC flag on the buffer. We cannot do xfs_buf_iowait()
+	 * here but a lock on the superblock buffer will block until iodone()
+	 * has completed.
+	 */
+	xfs_buf_lock(mp->m_sb_bp);
+	xfs_buf_unlock(mp->m_sb_bp);
+
 	xfs_log_unmount_write(mp);
 	xfs_log_unmount(mp);
 	xfs_uuid_unmount(mp);

fs/xfs/xfs_sync.c

@@ -359,6 +359,15 @@ xfs_quiesce_attr(
 	 * added an item to the AIL, thus flush it again.
 	 */
 	xfs_ail_push_all_sync(mp->m_ail);
+
+	/*
+	 * The superblock buffer is uncached and xfsaild_push() will lock and
+	 * set the XBF_ASYNC flag on the buffer. We cannot do xfs_buf_iowait()
+	 * here but a lock on the superblock buffer will block until iodone()
+	 * has completed.
+	 */
+	xfs_buf_lock(mp->m_sb_bp);
+	xfs_buf_unlock(mp->m_sb_bp);
 }
 
 static void