Old LOR between devfs & devfsmount resurfacing?
Attilio Rao
attilio at freebsd.org
Thu Feb 7 14:11:48 UTC 2008
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) @ /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.
Attilio
--
Peace can only be achieved by understanding - A. Einstein
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