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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