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

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.