On Thu, Feb 07, 2008 at 11:16:08AM +0100, Attilio Rao wrote:
> 2008/2/7, Kostik Belousov <email@example.com>:
> > 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 ?
> Just add LK_NOWITNESS for one of the lock involved in the lockinit().
Then, we loss the useful reports of the actual LORs later, isn't it ?