Re: kqueue LOR

On Tuesday 12 December 2006 13:43, Suleiman Souhlal wrote:
> Attilio Rao wrote:
> > 2006/12/12, Kostik Belousov <kostikbel_at_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) _at_ 
> >> /usr/src/sys/kern/kern_event.c:1547
> >> > >>+ 2nd 0xc45c22dc struct mount mtx (struct mount mtx) _at_
> >> > >>/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).
> 
> Why is memory barrier usage not encouraged? As you said, they can be used to 
reduce the number of atomic (LOCKed) operations, in some cases.
> 
> FWIW, Linux has rmb() (load mem barrier), wmb() (store mem barrier), mb() 
(load/store mem barrier), smp_rmb(), smp_wmb(), smp_mb() (mem barriers only 
needed on SMP), and barrier() (GCC barrier (__asm __volatile (:::"memory")) 
macros that I've personally found very useful.
> Admittedly, they are harder to use than atomic operations, but it might 
still worth having something similar.

Memory barriers just specify ordering, they don't ensure a cache flush so 
another CPU reads up to date values.  You can use memory barriers in 
conjunction with atomic operations on a variable to ensure that you can 
safely read other variables (which is what locks do).  For example, in this 
case IIUC, you have a race that is because there is shared state between two 
fields, one in the mount structure, and one in the ufs i-node.  Memory 
barriers alone won't prevent you from operating on those flags 
non-consistently.  That is, you have two memory locations in play here, and 
atomic ops only work on a single one.  There isn't an atomic op to do "read 
from memory location A, check flag B, and if it's true write C to memory 
location D".  Where would you put the membar in this case to ensure that the 
action always results in consistent behavior?

-- 
John Baldwin