linux-kernel - Re: v3.13-rc6+ regression (ARM board)

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Date:	Thu, 02 Jan 2014 12:52:56 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Krzysztof Hałasa <khalasa@...p.pl>
CC:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, Willy Tarreau <w@....eu>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: v3.13-rc6+ regression (ARM board)

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 <khalasa@...p.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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