Jan Kiszka wrote:
> Am 04.11.2010 00:18, Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Am 04.11.2010 00:11, Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> Am 03.11.2010 23:11, Jan Kiszka wrote:
>>>>>> Am 03.11.2010 23:03, Jan Kiszka wrote:
>>>>>>> But we not not always use atomic ops for manipulating status bits (but
>>>>>>> we do in other cases where this is no need - different story). This may
>>>>>>> fix the race:
>>>>>> Err, nonsense. As we manipulate xnsched::status also outside of nklock
>>>>>> protection, we must _always_ use atomic ops.
>>>>>>
>>>>>> This screams for a cleanup: local-only bits like XNHTICK or XNINIRQ
>>>>>> should be pushed in a separate status word that can then be safely
>>>>>> modified non-atomically.
>>>>> Second try to fix and clean up the sched status bits. Anders, please
>>>>> test.
>>>>>
>>>>> Jan
>>>>>
>>>>> diff --git a/include/nucleus/pod.h b/include/nucleus/pod.h
>>>>> index 01ff0a7..5987a1f 100644
>>>>> --- a/include/nucleus/pod.h
>>>>> +++ b/include/nucleus/pod.h
>>>>> @@ -277,12 +277,10 @@ static inline void xnpod_schedule(void)
>>>>> * context is active, or if we are caught in the middle of a
>>>>> * unlocked context switch.
>>>>> */
>>>>> -#if XENO_DEBUG(NUCLEUS)
>>>>> if (testbits(sched->status, XNKCOUT|XNINIRQ|XNSWLOCK))
>>>>> return;
>>>>> -#else /* !XENO_DEBUG(NUCLEUS) */
>>>>> - if (testbits(sched->status,
>>>>> - XNKCOUT|XNINIRQ|XNSWLOCK|XNRESCHED) != XNRESCHED)
>>>>> +#if !XENO_DEBUG(NUCLEUS)
>>>>> + if (!sched->resched)
>>>>> return;
>>>>> #endif /* !XENO_DEBUG(NUCLEUS) */
>>>> Having only one test was really nice here, maybe we simply read a
>>>> barrier before reading the status?
>>>>
>>> I agree - but the alternative is letting all modifications of
>>> xnsched::status use atomic bitops (that's required when folding all bits
>>> into a single word). And that should be much more costly, specifically
>>> on SMP.
>> What about issuing a barrier before testing the status?
>>
>
> The problem is not about reading but writing the status concurrently,
> thus it's not about the code you see above.
The bits are modified under nklock, which implies a barrier when
unlocked. Furthermore, an IPI is guaranteed to be received on the remote
CPU after this barrier, so, a barrier should be enough to see the
modifications which have been made remotely.
--
Gilles.
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