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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Howard Chu wrote:
> 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.

Regardless of why, that is just the simplest scenario I could think
of that would give us a test case. However...

Why not hold onto it? We sometimes do this in the kernel if we need
to take a lock that is incompatible with the lock already being held,
or if we discover we need to take a mutex which nests outside our
currently held lock in other paths. Ie to prevent deadlock.

Another reason might be because we will be running for a very long
time without requiring the lock. Or we might like to release it because
we expect a higher priority process to take it.

> > 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.

--
SUSE Labs, Novell Inc.
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