Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

[Frederic Weisbecker]

2014-08-14 3:57 GMT+02:00 Rik van Riel <riel@xxxxxxxxxx>:
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> On 08/13/2014 08:43 PM, Frederic Weisbecker wrote:
>> On Wed, Aug 13, 2014 at 05:03:24PM -0400, Rik van Riel wrote:
>>> --- a/kernel/time/posix-cpu-timers.c +++
>>> b/kernel/time/posix-cpu-timers.c @@ -272,22 +272,8 @@ static int
>>> posix_cpu_clock_get_task(struct task_struct *tsk, if
>>> (same_thread_group(tsk, current)) err =
>>> cpu_clock_sample(which_clock, tsk, &rtn); } else { -         unsigned
>>> long flags; -                struct sighand_struct *sighand; - -             /* -             *
>>> while_each_thread() is not yet entirely RCU safe, -           * keep
>>> locking the group while sampling process -            * clock for now. -
>>> */ -         sighand = lock_task_sighand(tsk, &flags); -             if (!sighand)
>>> -                    return err; - if (tsk == current ||
>>> thread_group_leader(tsk)) err =
>>> cpu_clock_sample_group(which_clock, tsk, &rtn); - -
>>> unlock_task_sighand(tsk, &flags); }
>>
>> I'm worried about such lockless solution based on RCU or read
>> seqcount because we lose the guarantee that an update is
>> immediately visible by all subsequent readers.
>>
>> Say CPU 0 updates the thread time and both CPU 1 and CPU 2 right
>> after that call clock_gettime(), with the spinlock we were
>> guaranteed to see the new update. Now with a pure seqlock read
>> approach, we guarantee a read sequence coherency but we don't
>> guarantee the freshest update result.
>>
>> So that looks like a source of non monotonic results.
>
> Which update are you worried about, specifically?
>
> The seq_write_lock to update the usage stat in p->signal will lock out
> the seqlock read side used to check those results.
>
> Is there another kind of thing read by cpu_clock_sample_group that you
> believe is not excluded by the seq_lock?

I mean the read side doesn't use a lock with seqlocks. It's only made
of barriers and sequence numbers to ensure the reader doesn't read
some half-complete update. But other than that it can as well see the
update n - 1 since barriers don't enforce latest results.
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