On 01/02/2014 12:42 PM, Stephen Boyd wrote:
> On 01/02/14 12:30, John Stultz wrote:
>> On 01/02/2014 12:03 PM, John Stultz wrote:
>>> On 01/02/2014 11:38 AM, Linus Torvalds wrote:
>>>> On Thu, Jan 2, 2014 at 4:07 AM, Krzysztof Hałasa <khal...@piap.pl> wrote:
>>>>> This means these two commits don't like each other:
>>>>>
>>>>> seqcount: Add lockdep functionality to seqcount/seqlock structures
>>>>> sched_clock: Use seqcount instead of rolling our own
>>>> Does something like this fix it for you?
>>>>
>>>> --- a/kernel/time/sched_clock.c
>>>> +++ b/kernel/time/sched_clock.c
>>>> @@ -36,6 +36,7 @@ core_param(irqtime, irqtime, int, 0400);
>>>>
>>>> static struct clock_data cd = {
>>>> .mult = NSEC_PER_SEC / HZ,
>>>> + .seq = SEQCNT_ZERO(cd.seq),
>>>> };
>>>>
>>>> static u64 __read_mostly sched_clock_mask;
>>>>
>>>> (The above is not even compile-tested, because x86 doesn't use
>>>> GENERIC_SCHED_CLOCK. So I did the patch blindly, but I think you get
>>>> the idea..)
>>> Sheesh. Just finishing up holiday email backlog and Linus already has a
>>> fix. :)
>>>
>>> This looks like it should fix the issue, and does build for me.
>>>
>>> Assuming it works for Krzysztof,
>> So something else may be at play. Even with Linus' patch I reproduced a
>> similar hang here.
>>
>> Still chasing it down, but it looks like a seqlock deadlock where we're
>> calling read while holding the lock.
>>
> Do you have tracing enabled? When I moved this code over to use
> seqcounts it relied on the fact that the compiler wouldn't be generating
> any function calls to the tracing code. Before seqcounts got lockdep
> support it all collapsed down into sched_clock() due to the use of
> inline on the seqlock API.
Hrm. I have tracing compiled in, I'll see if disabling it avoids the issue.
If this is the problem, I'm guessing we may need to change it to use
read_seqcount_begin_no_lockdep() then.
But I still don't have a clear sense of exactly whats happening yet.
thanks
-john
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