Re: Old LOR between devfs & devfsmount resurfacing?

2008/2/7, Kostik Belousov <kostikbel_at_gmail.com>:
> 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, ...);

No, I really want a general WITNESS support for this (as I also think
that having something more fine grained than BLESSING will break all
concerns jhb and me are considering now).
A simple way to do it would mean hard-coding file and line in a
witness table. While file is ok, line makes trouble so we should find
an alternative way to do this. Otherwise we can consider skiping
checks for a whole function, this should be not so difficult to
achive.

I need to think more about this.

Attilio


-- 
Peace can only be achieved by understanding - A. Einstein