On 06/19/2014 01:52 PM, Waiman Long wrote:
> On 06/19/2014 12:51 PM, Chris Mason wrote:
>> On 06/18/2014 11:21 PM, Waiman Long wrote:
>>> On 06/18/2014 10:11 PM, Chris Mason wrote:
>>>> On 06/18/2014 10:03 PM, Marc Dionne wrote:
>>>>> On Wed, Jun 18, 2014 at 8:41 PM, Marc
>>>>> Dionne<marc.c.dio...@gmail.com> wrote:
>>>>>> On Wed, Jun 18, 2014 at 8:08 PM, Waiman Long<waiman.l...@hp.com>
>>>>>> wrote:
>>>>>>> On 06/18/2014 08:03 PM, Marc Dionne wrote:
>>>>> And for an additional data point, just removing those
>>>>> CONFIG_DEBUG_LOCK_ALLOC ifdefs looks like it's sufficient to prevent
>>>>> the symptoms when lockdep is not enabled.
>>>> Ok, somehow we've added a lock inversion here that wasn't here before.
>>>> Thanks for confirming, I'll nail it down.
>>>>
>>>> -chris
>>>>
>>> I am pretty sure that the hangup is caused by the following kind of code
>>> fragment in the locking.c file:
>>>
>>> if (eb->lock_nested) {
>>> read_lock(&eb->lock);
>>> if (eb->lock_nested&& current->pid ==
>>> eb->lock_owner) {
>>>
>>> Is it possible to do the check without taking the read_lock?
>> I think you're right, we haven't added any new recursive takers of the
>> lock. The path where we are deadlocking has an extent buffer that isn't
>> in the path yet locked. I think we're taking the read lock while that
>> one is write locked.
>>
>> Reworking the nesting a big here.
>>
>> -chris
>
> I would like to take back my comments. I took out the read_lock, but the
> process still hang while doing file activities on btrfs filesystem. So
> the problem is trickier than I thought. Below are the stack backtraces
> of some of the relevant processes.
>
You weren't wrong, but it was also the tree trylock code. Our trylocks
only back off if the blocking lock is held. btrfs_next_leaf needs it to
be a true trylock. The confusing part is this hasn't really changed,
but one of the callers must be a spinner where we used to have a blocker.
-chris
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