On Thursday 07 February 2008 09:11:46 am Attilio Rao wrote:
> 2008/2/7, Kostik Belousov :
> > On Thu, Feb 07, 2008 at 01:21:09PM +0100, Attilio Rao wrote:
> > > 2008/2/7, Kostik Belousov :
> > > > On Thu, Feb 07, 2008 at 12:04:28PM +0100, Attilio Rao wrote:
> > > > > 2008/2/7, Kostik Belousov :
> > > > > > On Thu, Feb 07, 2008 at 11:16:08AM +0100, Attilio Rao wrote:
> > > > > > > 2008/2/7, 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 ?
> > > > > > >
> > > > > > > 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.


I think allowing a flag is fine, just as you can specify MTX_QUIET to quiet
KTR logs in specific mtx_lock() instances. You would specify LK_NOWITNESS or
some such and have it not do a witness_checkorder() in that case.

--
John Baldwin
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