Re: Nasty non-recursive lockmgr panic on softdep only enabled UFS partition when filesystem full

On Tue, May 03, 2011 at 11:58:49PM -0700, Garrett Cooper wrote:
> On Tue, May 3, 2011 at 11:42 PM, Garrett Cooper <yanegomi_at_gmail.com> wrote:
> > On Tue, May 3, 2011 at 10:59 PM, Kirk McKusick <mckusick_at_mckusick.com> wrote:
> >>> Date: Tue, 3 May 2011 22:40:26 -0700
> >>> Subject: Nasty non-recursive lockmgr panic on softdep only enabled UFS
> >>>  partition when filesystem full
> >>> From: Garrett Cooper <yanegomi_at_gmail.com>
> >>> To: Jeff Roberson <jeff_at_freebsd.org>,
> >>>         Marshall Kirk McKusick <mckusick_at_mckusick.com>
> >>> Cc: FreeBSD Current <freebsd-current_at_freebsd.org>
> >>>
> >>> Hi Jeff and Dr. McKusick,
> >>>     Ran into this panic when /usr ran out of space doing a make
> >>> universe on amd64/r221219 (it took ~15 minutes for the panic to occur
> >>> after the filesystem ran out of space -- wasn't quite sure what it was
> >>> doing at the time):
> >>>
> >>> ...
> >>>
> >>>     Let me know what other commands you would like for me to run in kgdb.
> >>> Thanks,
> >>> -Garrett
> >>
> >> You did not indicate whether you are running an 8.X system or a 9-current
> >> system. It would be helpful to know that.
> >
> > I've actually been running CURRENT for a few years now, but you're right --
> > I didn't mention that part.
> >
> >> Jeff thinks that there may be a potential race in the locking code for
> >> softdep_request_cleanup. If so, this patch for 9-current should fix it:
> >>
> >> Index: ffs_softdep.c
> >> ===================================================================
> >> --- ffs_softdep.c       (revision 221385)
> >> +++ ffs_softdep.c       (working copy)
> >> _at__at_ -11380,7 +11380,8 _at__at_
> >>                                continue;
> >>                        }
> >>                        MNT_IUNLOCK(mp);
> >> -                       if (vget(lvp, LK_EXCLUSIVE | LK_INTERLOCK, curthread)) {
> >> +                       if (vget(lvp, LK_EXCLUSIVE | LK_NOWAIT | LK_INTERLOCK,
> >> +                           curthread)) {
> >>                                MNT_ILOCK(mp);
> >>                                continue;
> >>                        }
> >>
> >> If you are running an 8.X system, hopefully you will be able to apply it.
> >
> >    I've applied it, rebuilt and installed the kernel, and trying to
> > repro the case again. Will let you know how things go!
> 
>     Happened again with the change. It's really easy to repro:
> 
> 1. Get a filesystem with UFS+SU
> 2. Execute something that does a large number of small writes to a partition.
> 3. 'dd if=/dev/zero of=FOO bs=10m' on the same partition
> 
>     The kernel will panic with the issue I discussed above.
> Thanks!

Jeff' change is required to avoid LORs, but it is not sufficient to
prevent recursion. We must skip the vnode supplied as a parameter to
softdep_request_cleanup(). Theoretically, other vnodes might be also
locked by curthread, thus I think the change below is needed. Try this.

diff --git a/sys/ufs/ffs/ffs_softdep.c b/sys/ufs/ffs/ffs_softdep.c
index a6d4441..25fa5d6 100644
--- a/sys/ufs/ffs/ffs_softdep.c
+++ b/sys/ufs/ffs/ffs_softdep.c
_at__at_ -11380,7 +11380,9 _at__at_ retry:
 				continue;
 			}
 			MNT_IUNLOCK(mp);
-			if (vget(lvp, LK_EXCLUSIVE | LK_INTERLOCK, curthread)) {
+			if (VOP_ISLOCKED(lvp) ||
+			    vget(lvp, LK_EXCLUSIVE | LK_INTERLOCK | LK_NOWAIT,
+			    curthread)) {
 				MNT_ILOCK(mp);
 				continue;
 			}