Re: Old LOR between devfs & devfsmount resurfacing?

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?
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


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