Date: Tue, 12 Dec 2006 15:43:28 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Kostik Belousov" <kostikbel@gmail.com> Cc: freebsd-stable@freebsd.org, freebsd-current@freebsd.org, Suleiman Souhlal <ssouhlal@freebsd.org>, V.Haisman@sh.cvut.cz, bde@freebsd.org, tegge@freebsd.org Subject: Re: kqueue LOR Message-ID: <3bbf2fe10612120643n6b4ad850oc857be46c48505cc@mail.gmail.com> In-Reply-To: <20061212101903.GF311@deviant.kiev.zoral.com.ua> References: <456950AF.3090308@sh.cvut.cz> <20061127092146.GA69556@deviant.kiev.zoral.com.ua> <457E6C06.1020405@FreeBSD.org> <20061212101903.GF311@deviant.kiev.zoral.com.ua>
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2006/12/12, Kostik Belousov <kostikbel@gmail.com>: > On Tue, Dec 12, 2006 at 12:44:54AM -0800, Suleiman Souhlal wrote: > > Kostik Belousov wrote: > > >On Sun, Nov 26, 2006 at 09:30:39AM +0100, V??clav Haisman wrote: > > > > > >>Hi, > > >>the attached lor.txt contains LOR I got this yesterday. It is FreeBSD 6.1 > > >>with relatively recent kernel, from last week or so. > > >> > > >>-- > > >>VH > > > > > > > > >>+lock order reversal: > > >>+ 1st 0xc537f300 kqueue (kqueue) @ /usr/src/sys/kern/kern_event.c:1547 > > >>+ 2nd 0xc45c22dc struct mount mtx (struct mount mtx) @ > > >>/usr/src/sys/ufs/ufs/ufs_vnops.c:138 > > >>+KDB: stack backtrace: > > >>+kdb_backtrace(c07f9879,c45c22dc,c07fd31c,c07fd31c,c080c7b2,...) at > > >>kdb_backtrace+0x2f > > >>+witness_checkorder(c45c22dc,9,c080c7b2,8a,c07fc6bd,...) at > > >>witness_checkorder+0x5fe > > >>+_mtx_lock_flags(c45c22dc,0,c080c7b2,8a,e790ba20,...) at > > >>_mtx_lock_flags+0x32 > > >>+ufs_itimes(c47a0dd0,c47a0e90,e790ba78,c060e1cc,c47a0dd0,...) at > > >>ufs_itimes+0x6c > > >>+ufs_getattr(e790ba54,e790baec,c0622af6,c0896f40,e790ba54,...) at > > >>ufs_getattr+0x20 > > >>+VOP_GETATTR_APV(c0896f40,e790ba54,c08a5760,c47a0dd0,e790ba74,...) at > > >>VOP_GETATTR_APV+0x3a > > >>+filt_vfsread(c4cf261c,6,c07f445e,60b,0,...) at filt_vfsread+0x75 > > >>+knote(c4f57114,6,1,1f30c2af,1f30c2af,...) at knote+0x75 > > >>+VOP_WRITE_APV(c0896f40,e790bbec,c47a0dd0,227,e790bcb4,...) at > > >>VOP_WRITE_APV+0x148 > > >>+vn_write(c45d5120,e790bcb4,c5802a00,0,c4b73a80,...) at vn_write+0x201 > > >>+dofilewrite(c4b73a80,1b,c45d5120,e790bcb4,ffffffff,...) at > > >>dofilewrite+0x84 > > >>+kern_writev(c4b73a80,1b,e790bcb4,8220c71,0,...) at kern_writev+0x65 > > >>+write(c4b73a80,e790bd04,c,c07d899c,3,...) at write+0x4f > > >>+syscall(3b,3b,bfbf003b,0,bfbfeae4,...) at syscall+0x295 > > >>+Xint0x80_syscall() at Xint0x80_syscall+0x1f > > >>+--- syscall (4, FreeBSD ELF32, write), eip = 0x2831d727, esp = > > >>0xbfbfea1c, ebp = 0xbfbfea48 --- > > > > > > > > >Thank you for the report. The LOR is caused by my commit into > > >sys/ufs/ufs/ufs_vnops.c, rev. 1.280. > > > > Is the mount lock really required, if all we're doing is a single read of a > > single word (mnt_kern_flags) (v_mount should be read-only for the whole > > lifetime of the vnode, I believe)? After all, reads of a single word are > > atomic on all our supported architectures. > > The only situation I see where there MIGHT be problems are forced unmounts, > > but I think there are bigger issues with those. > > Sorry for noticing this email only now. > > The problem is real with snapshotting. Ignoring > MNTK_SUSPEND/MNTK_SUSPENDED flags (in particular, reading stale value of > mnt_kern_flag) while setting IN_MODIFIED caused deadlock at ufs vnode > inactivation time. This was the big trouble with nfsd and snapshots. As > such, I think that precise value of mmnt_kern_flag is critical there, > and mount interlock is needed. This can be avoided using a memory barrier when setting flags. Even if memory barriers usage is not encouraged, some critical code should really use them replacing a mutex semantic (if that worths it). Attilio -- Peace can only be achieved by understanding - A. Einstein
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