Reduce probability for a deadlock that can occur when a snapshot inode is

updated by a process holding the snapshot lock.  Another process updating a
different inode in the same inodeblock will do copy on write checks and lock in
the opposite direction.

The snapshot code force a copy on write of these blocks manually (cf. start of
expunge_ufs[12]) and these inode blocks are later put on snapblklist.

This partial fix is to 'drain' the relevant ffs_copyonwrite() operation after
installing new snapblklist.  This is not a 100% solution since a failed block
allocation can cause implicit fsync() which might deadlock before the new
snapblklist has been installed.

1 files changed, 11 insertions, 0 deletions

diff --git a/sys/ufs/ffs/ffs_snapshot.c b/sys/ufs/ffs/ffs_snapshot.c
index 0dc07e9..98a1f23 100644
--- a/sys/ufs/ffs/ffs_snapshot.c
+++ b/sys/ufs/ffs/ffs_snapshot.c
@@ -681,6 +681,17 @@ out1:
 	VI_UNLOCK(devvp);
 	if (space != NULL)
 		FREE(space, M_UFSMNT);
+	/*
+	 * If another process is currently writing the buffer containing
+	 * the inode for this snapshot then a deadlock can occur. Drop
+	 * the snapshot lock until the buffer has been written.
+	 */
+	VOP_UNLOCK(vp, 0, td);
+	(void) bread(ip->i_devvp,
+		     fsbtodb(fs, ino_to_fsba(fs, ip->i_number)),
+		     (int) fs->fs_bsize, NOCRED, &nbp);
+	brelse(nbp);
+	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
 done:
 	FREE(copy_fs->fs_csp, M_UFSMNT);
 	bawrite(sbp);