Date: Wed, 13 Feb 2008 11:38:27 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Attilio Rao <attilio@freebsd.org>, Kostik Belousov <kostikbel@gmail.com>, Marcel Moolenaar <marcelm@juniper.net>, current@freebsd.org Subject: Re: Old LOR between devfs & devfsmount resurfacing? Message-ID: <200802131138.27471.jhb@freebsd.org> In-Reply-To: <3bbf2fe10802070611v6c7714b5y18bef10d586944c4@mail.gmail.com> References: <B269B28B-C66E-4AC6-A4D9-FBA378466F89@juniper.net> <20080207125252.GC57756@deviant.kiev.zoral.com.ua> <3bbf2fe10802070611v6c7714b5y18bef10d586944c4@mail.gmail.com>
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On Thursday 07 February 2008 09:11:46 am Attilio Rao wrote: > 2008/2/7, Kostik Belousov <kostikbel@gmail.com>: > > On Thu, Feb 07, 2008 at 01:21:09PM +0100, Attilio Rao wrote: > > > 2008/2/7, Kostik Belousov <kostikbel@gmail.com>: > > > > On Thu, Feb 07, 2008 at 12:04:28PM +0100, Attilio Rao wrote: > > > > > 2008/2/7, Kostik Belousov <kostikbel@gmail.com>: > > > > > > On Thu, Feb 07, 2008 at 11:16:08AM +0100, Attilio Rao wrote: > > > > > > > 2008/2/7, Kostik Belousov <kostikbel@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. 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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