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

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.

Attilio


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