Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)

[Howard Chu <hyc@symas.com>]

Nick Piggin wrote:
> OK, you believe that the mutex *must* be granted to a blocking thread
> at the time of the unlock. I don't think this is unreasonable from the
> wording (because it does not seem to be completely unambiguous english),
> however think about this -
>
> A realtime system with tasks A and B, A has an RT scheduling priority of
> 1, and B is 2. A and B are both runnable, so A is running. A takes a 
> mutex
> then sleeps, B runs and ends up blocked on the mutex. A wakes up and at
> some point it drops the mutex and then tries to take it again.
>
> What happens?
>
> I haven't programmed realtime systems of any complexity, but I'd think it
> would be undesirable if A were to block and allow B to run at this point.

But why does A take the mutex in the first place? Presumably because it 
is about to execute a critical section. And also presumably, A will not 
release the mutex until it no longer has anything critical to do; 
certainly it could hold it longer if it needed to.

If A still needed the mutex, why release it and reacquire it, why not 
just hold onto it? The fact that it is being released is significant.

> Now this has nothing to do with PI or SCHED_OTHER, so behaviour is 
> exactly
> determined by our respective interpretations of what it means for "the
> scheduling policy to decide which task gets the mutex".
>
> What have I proven? Nothing ;) but perhaps my question could be answered
> by someone who knows a lot more about RT systems than I.

In the last RT work I did 12-13 years ago, there was only one high 
priority producer task and it was never allowed to block. The consumers 
just kept up as best they could (multi-proc machine of course). I've 
seldom seen a need for many priority levels. Probably not much you can 
generalzie from this though.

-- 
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/