Re: fs: locks: WARNING: CPU: 16 PID: 4296 at fs/locks.c:236 locks_free_lock_context+0x10d/0x240()

[Sasha Levin]

On 01/14/2015 09:27 AM, Jeff Layton wrote:
> On Tue, 13 Jan 2015 17:50:45 -0500
> Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> 
>> On 01/13/2015 04:44 PM, Jeff Layton wrote:
>>> On Tue, 13 Jan 2015 00:11:37 -0500
>>> Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
>>>
>>>> Hey Jeff,
>>>>
>>>> While fuzzing with trinity inside a KVM tools guest running the latest -next
>>>> kernel, I've stumbled on the following spew:
>>>>
>>>> [  887.078606] WARNING: CPU: 16 PID: 4296 at fs/locks.c:236 locks_free_lock_context+0x10d/0x240()
>>>> [  887.079703] Modules linked in:
>>>> [  887.080288] CPU: 16 PID: 4296 Comm: trinity-c273 Not tainted 3.19.0-rc4-next-20150112-sasha-00053-g23c147e02e-dirty #1710
>>>> [  887.082229]  0000000000000000 0000000000000000 0000000000000000 ffff8804c9f4f8e8
>>>> [  887.083773]  ffffffff9154e0a6 0000000000000000 ffff8804cad98000 ffff8804c9f4f938
>>>> [  887.085280]  ffffffff8140a4d0 0000000000000001 ffffffff81bf0d2d ffff8804c9f4f988
>>>> [  887.086792] Call Trace:
>>>> [  887.087320] dump_stack (lib/dump_stack.c:52)
>>>> [  887.088247] warn_slowpath_common (kernel/panic.c:447)
>>>> [  887.089342] ? locks_free_lock_context (fs/locks.c:236 (discriminator 3))
>>>> [  887.090514] warn_slowpath_null (kernel/panic.c:481)
>>>> [  887.091629] locks_free_lock_context (fs/locks.c:236 (discriminator 3))
>>>> [  887.092782] __destroy_inode (fs/inode.c:243)
>>>> [  887.093817] destroy_inode (fs/inode.c:268)
>>>> [  887.094833] evict (fs/inode.c:574)
>>>> [  887.095808] iput (fs/inode.c:1503)
>>>> [  887.096687] __dentry_kill (fs/dcache.c:323 fs/dcache.c:508)
>>>> [  887.097683] ? _raw_spin_trylock (kernel/locking/spinlock.c:136)
>>>> [  887.098733] ? dput (fs/dcache.c:545 fs/dcache.c:648)
>>>> [  887.099672] dput (fs/dcache.c:649)
>>>> [  887.100552] __fput (fs/file_table.c:227)
>>>
>>> So, looking at this a bit more...
>>>
>>> It's clear that we're at the dput in __fput at this point. Much earlier
>>> in __fput, we call locks_remove_file to remove all of the locks that
>>> are associated with the file description.
>>>
>>> Evidently though, something didn't go right there. The two most likely
>>> scenarios to my mind are:
>>>
>>> A) a lock raced onto the list somehow after that point. That seems
>>> unlikely since presumably the fcheck should have failed at that point.
>>>
>>> ...or...
>>>
>>> B) the CPU that called locks_remove_file mistakenly thought that
>>> inode->i_flctx was NULL when it really wasn't (stale cache, perhaps?).
>>> That would make it skip trying to remove any flock locks.
>>>
>>> B seems more likely to me, and if it's the case then that would seem to
>>> imply that we need some memory barriers (or maybe some ACCESS_ONCE
>>> calls) in these codepaths. I'll have to sit down and work through it to
>>> see what makes the most sense.
>>>
>>> If your debugging seems to jive with this, then one thing that might be
>>> interesting would be to comment out these two lines in
>>> locks_remove_flock:
>>>
>>>         if (!file_inode(filp)->i_flctx)
>>>                 return;
>>>
>>> ...and see if it's still reproducible. That's obviously not a real fix
>>> for this problem, but it might help prove whether the above suspicion
>>> is correct.
>>
>> Removing those two lines makes the issue go away.
>>
>> I'm guessing that figuring out which filesystem we were abusing isn't
>> interesting anymore...
>>
> 
> Sigh. I've been trying to reproduce this today. I've set up two
> different KVM guests on two different hosts, and run trinity on both,
> and I can't seem to get this warning to pop.
> 
> Could you share what trinity command-line options you're using? Any
> other special setup I should be considering to reproduce it?
> 
> I was hoping to get it to reproduce so I could test out potential
> memory barrier fixes...

I don't think there's anything special about my setup here that can
trigger that, specially if it's not dependant on a filesystem in use.

I'm running trinity with: ./trinity -xsched_setattr -xsetpriority
-xunshare -xreboot -xshutdown -xnfsservctl -xclock_nanosleep -xuselib
-xperf_event_open -m --quiet --dangerous -C 400 -l off

If that doesn't end up helping, I'd be happy to test out fixes here,
it usually reproduces quickly.


Thanks,
Sasha
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