Re: Old LOR between devfs & devfsmount resurfacing?

On Thu, Feb 07, 2008 at 12:04:28PM +0100, Attilio Rao wrote:
> 2008/2/7, Kostik Belousov <kostikbel_at_gmail.com>:
> > On Thu, Feb 07, 2008 at 11:16:08AM +0100, Attilio Rao wrote:
> >  > 2008/2/7, Kostik Belousov <kostikbel_at_gmail.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) _at_ /nfs/freebsd/8.x/src/sys/kern/
> >  > >  > vfs_subr.c:2061
> >  > >  >  2nd 0xdfb014 devfsmount (devfsmount) _at_ /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 ?
> 
> Another solution would be to rewamp BLESSING option which allow to
> 'bless' some LORs.
> jhb and me, btw, didn't want to enable it because it could lead some
> less experienced developer to hide LORs under this label and this is
> something we want to avoid.

This LOR shall not be ignored globally. When real, it caused the easily
reproducable lockup of the machine.

It would be better to introduce some lockmgr flag to ignore _this_ locking.