Re: kqueue LOR

[Suleiman Souhlal]

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.

-- Suleiman
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