Re: [PATCH] xnsynch_requeue_sleeper: Return early when !XNSYNCH_PRIO

On 14.04.26 13:02, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
>> Von: "Jan Kiszka" <[email protected]>
>> Right, a prio adjustment of the lock-holder that is blocked on xddp will
>> not take place with PP - prio adjustments happen already on lock entry.
>> But your changes would still cause regressions to
>> xnsynch_requeue_sleeper.
>>
>> Let's step back: In the PI case, the lock holder will be on a FIFO queue
>> while waiting for messages. So it may not be woken up first if there are
>> multiple waiters - that is something we probably cannot change as we
>> have no information about other waiters in that queue and their urgency.
>>
>> Once woken up, my next question would be if the right prio is used from
>> there on until the lock is released. That should be checked by test
>> case.
>>
>> Now to clarify why your changes is not yet correct: A thread is blocked
>> on a PP mutex while holding a PI one. If someone tries to get the PI
>> lock while have a high prio than the PP mutex, the necessary propagation
>> of this prio to lock owner will no longer work. I'm not sure if we have
>> such a test case in Xenomai, we definitely should, and this one would
>> reveal the regression.
>>
>> Maybe this is already enough and correct:
>>
>> diff --git a/kernel/cobalt/synch.c b/kernel/cobalt/synch.c
>> index 70c05c1f49..33ef8ef183 100644
>> --- a/kernel/cobalt/synch.c
>> +++ b/kernel/cobalt/synch.c
>> @@ -963,7 +963,9 @@ void xnsynch_requeue_sleeper(struct xnthread *thread)
>> 	struct xnsynch *synch = thread->wchan;
>> 	struct xnthread *owner;
>>
>> -	XENO_BUG_ON(COBALT, !(synch->status & XNSYNCH_PRIO));
>> +	/* Nothing to do if waiting in a FIFO queue */
>> +	if ((synch->status & XNSYNCH_PRIO) == 0)
> 
> The comment is a bit misleading, IMHO.
> !XNSYNCH_PRIO does not guarantee that the synch object is a FIFO queue.

 * - XNSYNCH_PRIO causes the threads waiting for the resource to pend
 * in priority order. Otherwise, FIFO ordering is used (XNSYNCH_FIFO).

> 
>> +		return;
> 
> It passed my tests. I'll resend updates patches ASAP.
> 

Perfect!

Jan

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