kqueue LOR

Suleiman Souhlal ssouhlal at freebsd.org
Wed Dec 13 07:36:31 PST 2006

Kostik Belousov wrote:
> On Sun, Nov 26, 2006 at 09:30:39AM +0100, V??clav Haisman wrote:
>>the attached lor.txt contains LOR I got this yesterday. It is FreeBSD 6.1
>>with relatively recent kernel, from last week or so.
>>+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.

-- Suleiman
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