Re: LOR: vfs_mount.c (ufs) / msdosfs_vfsops.c (devfs)

On Mon, Feb 08, 2010 at 04:06:56PM +0100, Attilio Rao wrote:
> 2010/2/8 Attilio Rao <attilio_at_freebsd.org>:
> > 2010/2/8 Kostik Belousov <kostikbel_at_gmail.com>:
> >> On Mon, Feb 08, 2010 at 09:00:44AM -0500, John Baldwin wrote:
> >>> On Sunday 07 February 2010 11:00:32 am Bruce Cran wrote:
> >>> > Running -CURRENT from today, I unmounted the msdosfs filesystem on my
> >>> > phone and got the following LOR:
> >>> >
> >>> > lock order reversal:
> >>> >  1st 0xffffff00c51279f8 ufs (ufs) _at_ /usr/src/sys/kern/vfs_mount.c:1204
> >>> >  2nd 0xffffff010b892278 devfs (devfs) _at_
> >>> > /usr/src/sys/modules/msdosfs/../../fs/msdosfs/msdosfs_vfsops.c:944
> >>> > KDB: stack backtrace:
> >>> > db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
> >>> > _witness_debugger() at witness_debugger+0x2e
> >>> > witness_checkorder() at witness_checkorder+0x81e
> >>> > __lockmgr_args() at __lockmgr_args+0xd11
> >>> > vop_stdlock() at vop_stdlock+0x39
> >>> > VOP_LOCK1_APV() VOP_LOCK1_APV+0x9b
> >>> > _vn_lock() at _vn_lock+0x47
> >>> > msdosfs_sync() at msdosfs_sync+0x227
> >>> > dounmount() at dounmount+0x2ca
> >>> > unmount() at unmount+0x216
> >>> > syscall() at syscall+0x2a2
> >>> > Xfast_syscall() at Xfast_syscall+0xe1
> >>> > --- syscall (22, FreeBSD ELF64, unmount), rip = 0x8006a1e3c, rsp =
> >>> > 0x7fffffffe3a8, rbp = 0x800c08010 ---
> >>>
> >>> This is due to holding a lock on the coveredvp vnode for most of unmount(2).
> >>> Probably it should not be held for all of that.  Perhaps it is safe to just
> >>> keep the vnode referenced instead, or could the handling for coveredvp just
> >>> move to the end of the function where it is now vput?
> >>
> >> Among other things, holding vnode lock on covered vnode prevents parallel
> >> unmounts of the same mount point.
> >
> > Uhm, I think that this should be hanlded by MNTK_UNMOUNT already (and
> > thus stopping forced unmounts too).
> 
> In other words probabilly keeping coveredvnode held until MNTK_UNMOUNT
> and then refcounting it should be fine.

Actually, the coveredvp then might be reclaimed ? Failed unmount then
cannot recover.