Re: Old LOR between devfs & devfsmount resurfacing?

On Thu, Feb 07, 2008 at 01:21:09PM +0100, Attilio Rao wrote:
> 2008/2/7, Kostik Belousov <kostikbel_at_gmail.com>:
> > 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.
> 
> flag to pass where?
To the lockmgr itself at the point of aquisition, like
	lockmgr(&lk, LK_EXCLUSIVE | LK_INTERLOCK | LK_NOWARN, &interlk, ...);

> Really I think this will need some witness tweaking.
> After all a mechanism for ignoring specific LOR (happening at
> specified file-line) is not a bad idea.
> 
> Attilio
> 
> 
> -- 
> Peace can only be achieved by understanding - A. Einstein