1) Fix a deadlock in the VFS:

- threadA runs vfs_rel(mp1)
- threadB does unmount the mp1 fs, sets MNTK_UNMOUNT and drop MNT_ILOCK()
- threadA runs vfs_busy(mp1) and, as long as, MNTK_UNMOUNT is set, sleeps
  waiting for threadB to complete the unmount
- threadB, in vfs_mount_destroy(), finds mnt_lock > 0 and sleeps waiting
  for the refcount to expire.

Fix the deadlock by adding a flag called MNTK_REFEXPIRE which signals the
unmounter is waiting for mnt_ref to expire.
The vfs_busy contenders got awake, fails, and if they retry the
MNTK_REFEXPIRE won't allow them to sleep again.

2) Simplify significantly the code of vfs_mount_destroy() trimming
   unnecessary codes:
   - as long as any reference exited, it is no-more possible to have
     write-op (primarty and secondary) in progress.
   - it is no needed to drop and reacquire the mount lock.
   - filling the structures with dummy values is unuseful as long as
     it is going to be freed.

Tested by:	pho, Andrea Barberio <insomniac at slackware dot it>
Discussed with:	kib

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 16373b1..eb02693 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -345,7 +345,7 @@ vfs_busy(struct mount *mp, int flags)
 	MNT_ILOCK(mp);
 	MNT_REF(mp);
 	if (mp->mnt_kern_flag & MNTK_UNMOUNT) {
-		if (flags & MBF_NOWAIT) {
+		if (flags & MBF_NOWAIT || mp->mnt_kern_flag & MNTK_REFEXPIRE) {
 			MNT_REL(mp);
 			MNT_IUNLOCK(mp);
 			return (ENOENT);