linux-kernel - Re: [RFC][PATCH 3/5] timekeeping: Avoid possible deadlock from clock_was_set

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Date:	Mon, 16 Dec 2013 21:15:18 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...nel.org>, stable <stable@...r.kernel.org>
Subject: Re: [RFC][PATCH 3/5] timekeeping: Avoid possible deadlock from clock_was_set_delayed

On 12/12/2013 11:13 AM, John Stultz wrote:
> On 12/12/2013 11:05 AM, Sasha Levin wrote:
>> On 12/12/2013 01:59 PM, John Stultz wrote:
>>> On 12/12/2013 10:32 AM, Sasha Levin wrote:
>>>> On 12/12/2013 11:34 AM, Sasha Levin wrote:
>>>>> On 12/11/2013 02:11 PM, John Stultz wrote:
>>>>>> As part of normal operaions, the hrtimer subsystem frequently calls
>>>>>> into the timekeeping code, creating a locking order of
>>>>>>     hrtimer locks -> timekeeping locks
>>>>>>
>>>>>> clock_was_set_delayed() was suppoed to allow us to avoid deadlocks
>>>>>> between the timekeeping the hrtimer subsystem, so that we could
>>>>>> notify the hrtimer subsytem the time had changed while holding
>>>>>> the timekeeping locks. This was done by scheduling delayed work
>>>>>> that would run later once we were out of the timekeeing code.
>>>>>>
>>>>>> But unfortunately the lock chains are complex enoguh that in
>>>>>> scheduling delayed work, we end up eventually trying to grab
>>>>>> an hrtimer lock.
>>>>>>
>>>>>> Sasha Levin noticed this in testing when the new seqlock lockdep
>>>>>> enablement triggered the following (somewhat abrieviated) message:
>>>>> [snip]
>>>>>
>>>>> This seems to work for me, I don't see the lockdep spew anymore.
>>>>>
>>>>>       Tested-by: Sasha Levin <sasha.levin@...cle.com>
>>>> I think I spoke too soon.
>>>>
>>>> It took way more time to reproduce than previously, but I got:
>>>>
>>>>
>>>> -> #1 (&(&pool->lock)->rlock){-.-...}:
>>>> [ 1195.578519]        [<ffffffff81194803>] validate_chain+0x6c3/0x7b0
>>>> [ 1195.578519]        [<ffffffff81194d9d>] __lock_acquire+0x4ad/0x580
>>>> [ 1195.578519]        [<ffffffff81194ff2>] lock_acquire+0x182/0x1d0
>>>> [ 1195.578519]        [<ffffffff843b0760>] _raw_spin_lock+0x40/0x80
>>>> [ 1195.578519]        [<ffffffff81153e0e>] __queue_work+0x14e/0x3f0
>>>> [ 1195.578519]        [<ffffffff81154168>] queue_work_on+0x98/0x120
>>>> [ 1195.578519]        [<ffffffff81161351>]
>>>> clock_was_set_delayed+0x21/0x30
>>>> [ 1195.578519]        [<ffffffff811c4b41>] do_adjtimex+0x111/0x160
>>>> [ 1195.578519]        [<ffffffff811360e3>] SYSC_adjtimex+0x43/0x80
>>>> [ 1195.578519]        [<ffffffff8113612e>] SyS_adjtimex+0xe/0x10
>>>> [ 1195.578519]        [<ffffffff843baed0>] tracesys+0xdd/0xe2
>>>> [ 1195.578519]
>>> Are you sure you have that patch applied?
>>>
>>> With it we shouldn't be calling clock_was_set_delayed() from
>>> do_adjtimex().
>> Hm, It seems that there's a conflict there that wasn't resolved
>> properly. Does this patch
>> depend on anything else that's not currently in -next?
> Oh yes, sorry, I didn't cc you on the entire patch set. Apologies!
>
> You'll probably want to grab the two previous patches:
> https://lkml.org/lkml/2013/12/11/479
> https://lkml.org/lkml/2013/12/11/758

Just wanted to follow up here. Did you happen to get a chance to try to
reproduce w/ the three patch patchset?

I'm hoping to submit them to Ingo tomorrow, and want to make sure I've
got your tested-by.

thanks
-john

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