Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework

[Andy Lutomirski]

On Thu, May 22, 2014 at 6:09 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, May 22, 2014 at 02:58:18PM +0200, Peter Zijlstra wrote:
>> ---
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 4ea7b3f1a087..a5da85fb3570 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -546,12 +546,38 @@ static bool set_nr_and_not_polling(struct task_struct *p)
>>       struct thread_info *ti = task_thread_info(p);
>>       return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
>>  }
>> +
>> +/*
>> + * Atomically set TIF_NEED_RESCHED if TIF_POLLING_NRFLAG is set.
>> + */
>> +static bool set_nr_if_polling(struct task_struct *p)
>> +{
>> +     struct thread_info *ti = task_thread_info(p);
>> +     typeof(ti->flags) old, val = ti->flags;
>> +
>> +     for (;;) {
>> +             if (!(val & _TIF_POLLING_NRFLAG))
>> +                     return false;
>> +             if (val & _TIF_NEED_RESCHED)
>> +                     return true;
>
> Hmm, I think this is racy, false would be safer. If its already set we
> might already be past the sched_ttwu_pending() invocation, while if its
> clear and we're the one to set it, we're guaranteed not.
>
>> +             old = cmpxchg(&ti->flags, val, val | _TIF_NEED_RESCHED);
>> +             if (old == val)
>> +                     return true;
>> +             val = old;
>> +     }
>> +}

Do you have an updated patch?  After fixing the MIME flow damage
(sigh), it doesn't apply to sched/core, which is my best guess for
what it's based on.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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