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

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Date:	Thu, 12 Dec 2013 14:05:15 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	John Stultz <john.stultz@...aro.org>,
	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 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?


Thanks,
Sasha

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