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

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

Howard Chu wrote:
> David Schwartz wrote:

> The time at which the decision takes effect is immaterial; the point is 
> that the decision can only be made from the set of options available at 
> time T.
>
> Per your analogy, if a new bid comes in at time T+1, it can't have any 
> effect on which of the bids shall be accepted.
>
> >     Third, there's the ambiguity of the standard. It says the "sceduling
> > policy" shall decide, not that the scheduler shall decide. If the 
> > policy is
> > to make a conditional or delayed decision, that is still perfectly valid
> > policy. "Whichever thread requests it first" is a valid scheduler policy.
>
>
> I am not debating what the policy can decide. Merely the set of choices 
> from which it may decide.

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.

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.

Nick

--
SUSE Labs, Novell Inc.
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