Old LOR between devfs & devfsmount resurfacing?

[Kostik Belousov]

On Wed, Feb 06, 2008 at 11:11:06AM -0800, Marcel Moolenaar wrote:
> All,
> 
> I just ran into the following LOR after upgrading my PowerPC box:
> 
> lock order reversal:
>  1st 0xdbee94 devfs (devfs) @ /nfs/freebsd/8.x/src/sys/kern/ 
> vfs_subr.c:2061
>  2nd 0xdfb014 devfsmount (devfsmount) @ /nfs/freebsd/8.x/src/sys/fs/ 
> devfs/devfs_vnops.c:201
> KDB: stack backtrace:
> 0xdc0febc8: at kdb_backtrace+0x4c
> 0xdc0febd8: at witness_checkorder+0x704
> 0xdc0fec28: at _sx_xlock+0x8c
> 0xdc0fec48: at devfs_allocv+0x138
> 0xdc0fec88: at devfs_root+0x5c
> 0xdc0fecb8: at set_rootvnode+0x44
> 0xdc0fece8: at vfs_mountroot+0x344
> 0xdc0fed48: at start_init+0x88
> 0xdc0feda8: at fork_exit+0xb4
> 0xdc0fedc8: at fork_trampoline+0xc
> KDB: enter: witness_checkorder
> [thread pid 1 tid 100001 ]
> Stopped at      0x28ee68:       addi    r0, r0, 0x0
> 
> It seems that this is a LOR reported in 2006 and fixed
> in 2006 as well. Do other people see this too, or should
> I suspect my sources?

I believe this is a false positive, caused by the way the witness works.
Attilio recently added the witness support for the lockmgr, that caused
this and at least two more LORs to be printed on startup.

Correct lock order is devfs vnode -> devfs mount sx lock. When
allocating new devfs vnode, see devfs_allocv(), the newly created
vnode is locked while devfs mount lock already held (see line 250 of
fs/devfs/devfs_vnops.c). Nonetheless, this cannot cause deadlock since
no other thread can find the new vnode, and thus perform the other lock
order for this vnode lock.

The fix is to shut the witness in this particular case. Attilio, how to
do this ?
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