git: dbc689cdef0c - main - vfs: use vn_lock_pair to avoid establishing an ordering on mount

[Mateusz Guzik]

The branch main has been updated by mjg:

URL: https://cgit.FreeBSD.org/src/commit/?id=dbc689cdef0cc8ff11171642cdcf107dfbc3fb41

commit dbc689cdef0cc8ff11171642cdcf107dfbc3fb41
Author:     Mateusz Guzik <mjg at FreeBSD.org>
AuthorDate: 2021-08-18 21:14:16 +0000
Commit:     Mateusz Guzik <mjg at FreeBSD.org>
CommitDate: 2021-08-20 17:52:24 +0000

    vfs: use vn_lock_pair to avoid establishing an ordering on mount
    
    This fixes some of the LORs seen on mount/unmount.
    
    Complete fix will require taking care of unmount as well.
    
    Reviewed by:    kib
    Tested by:      pho (previous version)
    Sponsored by:   Rubicon Communications, LLC ("Netgate")
    Differential Revision:  https://reviews.freebsd.org/D31611
---
 sys/kern/vfs_mount.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/sys/kern/vfs_mount.c b/sys/kern/vfs_mount.c
index 3c546392b213..92e70e45d46e 100644
--- a/sys/kern/vfs_mount.c
+++ b/sys/kern/vfs_mount.c
@@ -1191,19 +1191,28 @@ vfs_domount_first(
 		mp->mnt_kern_flag &= ~MNTK_ASYNC;
 	MNT_IUNLOCK(mp);
 
-	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
-	cache_purge(vp);
 	VI_LOCK(vp);
-	vp->v_iflag &= ~VI_MOUNT;
+	cache_purge(vp);
 	vn_irflag_set_locked(vp, VIRF_MOUNTPOINT);
 	vp->v_mountedhere = mp;
 	VI_UNLOCK(vp);
+
+	/*
+	 * We need to lock both vnodes.
+	 *
+	 * Use vn_lock_pair to avoid establishing an ordering between vnodes
+	 * from different filesystems.
+	 */
+	vn_lock_pair(vp, false, newdp, false);
+
+	VI_LOCK(vp);
+	vp->v_iflag &= ~VI_MOUNT;
+	VI_UNLOCK(vp);
 	/* Place the new filesystem at the end of the mount list. */
 	mtx_lock(&mountlist_mtx);
 	TAILQ_INSERT_TAIL(&mountlist, mp, mnt_list);
 	mtx_unlock(&mountlist_mtx);
 	vfs_event_signal(NULL, VQ_MOUNT, 0);
-	vn_lock(newdp, LK_EXCLUSIVE | LK_RETRY);
 	VOP_UNLOCK(vp);
 	EVENTHANDLER_DIRECT_INVOKE(vfs_mounted, mp, newdp, td);
 	VOP_UNLOCK(newdp);