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Message-ID: <43D93542.9000809@yahoo.com.au>
Date: Fri, 27 Jan 2006 07:46:58 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Howard Chu <hyc@symas.com>
CC: davids@webmaster.com,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)
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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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