Date: Thu, 7 Feb 2008 11:16:08 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Kostik Belousov" <kostikbel@gmail.com> Cc: Marcel Moolenaar <marcelm@juniper.net>, current@freebsd.org Subject: Re: Old LOR between devfs & devfsmount resurfacing? Message-ID: <3bbf2fe10802070216idd5206ey7a66c0873311e66c@mail.gmail.com> In-Reply-To: <20080207045015.GW57756@deviant.kiev.zoral.com.ua> References: <B269B28B-C66E-4AC6-A4D9-FBA378466F89@juniper.net> <20080207045015.GW57756@deviant.kiev.zoral.com.ua>
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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(). Attilio -- Peace can only be achieved by understanding - A. Einstein
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