[RFC] xfs: wait for the write of the superblock on unmount

[tinguely@xxxxxxx]

Sorry, I have been distracted away from this regression. This was previously
titled "xfs: synchronously write the superblock on unmount".

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.

The log write of the superblock is important for possible recovery, but a
second syncronous write of the same superblock seems redundant. Would just
waiting for the iodone() of the log write before tearing down the log be
enough?

Signed-off-by: Mark Tinguely <tinguely@xxxxxxx>

---
 fs/xfs/xfs_mount.c |    8 ++++++++
 1 file changed, 8 insertions(+)

Index: b/fs/xfs/xfs_mount.c
===================================================================
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1457,6 +1457,7 @@ void
 xfs_unmountfs(
        struct xfs_mount        *mp)
 {
+       struct xfs_buf          *bp = mp->m_sb_bp;
        __uint64_t              resblks;
        int                     error;
 
@@ -1529,6 +1530,13 @@ xfs_unmountfs(
        xfs_ail_push_all_sync(mp->m_ail);
        xfs_wait_buftarg(mp->m_ddev_targp);
 
+       /*
+        * Wait for the superblock to be written out. The superblock buffer
+        * will remain locked until the iodone().
+        */
+       xfs_buf_lock(bp);
+       xfs_buf_unlock(bp);
+
        xfs_log_unmount_write(mp);
        xfs_log_unmount(mp);
        xfs_uuid_unmount(mp);