PR kern/161016 Need to force sync(2) before umounting UFS1 filesystems?

[Kirk McKusick]

> Date: Sun, 25 Sep 2011 12:07:18 -0700
> From: Garrett Cooper <yanegomi at gmail.com>
> To: lev at freebsd.org
> Cc: freebsd-fs at freebsd.org, Xin LI <delphij at freebsd.org>, current at freebsd.org
> Subject: Re: Need to force sync(2) before umounting UFS1 filesystems?
> 
> 2011/9/25 Lev Serebryakov <lev at freebsd.org>:
> > Hello, Garrett.
> > You wrote 25 =D3=C5=CE=D4=D1=C2=D2=D1 2011 =C7., 12:06:05:
> >
> >> =9A =9A Talking to Xin yesterday, he was convinced that this was a
> >> filesystem//kern bug. Before I file a PR, I'm wondering if anyone else
> >> has seen this issue..
> > =9AYes, and I posted message about it in embedded@ (Message-ID
> > <1175277342.20110821215629 at serebryakov.spb.ru>), I've got additional
> > question from Warner Losh about base (underlying) file system, without
> > any additional reaction.
> 
> Thanks for the comments Adrian and Lev! I've filed PR 161016 to track
> the issue, because it might be due to changes in the SU code, md, or a
> subtle race condition in umount (highly unlikely, but it's been
> noted).
> -Garrett
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I have taken responsibility for working on this bug report (PR kern/161016).

I propose the following change to correct it:

Index: sys/kern/vfs_mount.c
===================================================================
--- sys/kern/vfs_mount.c	(revision 225807)
+++ sys/kern/vfs_mount.c	(working copy)
@@ -1227,18 +1227,6 @@
 		mp->mnt_kern_flag |= MNTK_UNMOUNTF;
 	error = 0;
 	if (mp->mnt_lockref) {
-		if ((flags & MNT_FORCE) == 0) {
-			mp->mnt_kern_flag &= ~(MNTK_UNMOUNT | MNTK_NOINSMNTQ |
-			    MNTK_UNMOUNTF);
-			if (mp->mnt_kern_flag & MNTK_MWAIT) {
-				mp->mnt_kern_flag &= ~MNTK_MWAIT;
-				wakeup(mp);
-			}
-			MNT_IUNLOCK(mp);
-			if (coveredvp)
-				VOP_UNLOCK(coveredvp, 0);
-			return (EBUSY);
-		}
 		mp->mnt_kern_flag |= MNTK_DRAINING;
 		error = msleep(&mp->mnt_lockref, MNT_MTX(mp), PVFS,
 		    "mount drain", 0);

The things to check for are:

1) That it fixes the EBUSY on unmount.

2) That it does not cause unmount to hang.

I would appreciate feedback as to whether this fix helps.

	Kirk McKusick