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

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

Howard Chu wrote:
> Nick Piggin wrote:
> 
>> Howard Chu wrote:
>>
>>> The SUSv3 text seems pretty clear. It says "WHEN 
>>> pthread_mutex_unlock() is called, ... the scheduling policy SHALL 
>>> decide ..." It doesn't say MAY, and it doesn't say "some undefined 
>>> time after the call." There is nothing optional or 
>>> implementation-defined here. The only thing that is not explicitly 
>>> stated is what happens when there are no waiting threads; in that 
>>> case obviously the running thread can continue running.
>>>
>>
>> But it doesn't say the unlocking thread must yield to the new mutex
>> owner, only that the scheduling policy shall determine the which
>> thread aquires the lock.
> 
> 
> True, the unlocking thread doesn't have to yield to the new mutex owner 
> as a direct consequence of the unlock. But logically, if the unlocking 
> thread subsequently calls mutex_lock, it must block, because some other 
> thread has already been assigned ownership of the mutex.
> 
>> It doesn't say that decision must be made immediately, either (eg.
>> it could be made as a by product of which contender is chosen to run
>> next).
> 
> 
> A straightforward reading of the language here says the decision happens 
> "when pthread_mutex_unlock() is called" and not at any later time. There 
> is nothing here to support your interpretation.
> 

OK, so what happens if my scheduling policy decides _right then_, that
the next _running_ thread that was being blocked on or tries to aquire
the mutex, is the next owner?

This is the logical way for a *scheduling* policy to determine which
thread gets the mutex. I don't know any other way that the scheduling
policy could determine the next thread to get the mutex.

>>
>> I think the intention of the wording is that for deterministic policies,
>> it is clear that the waiting threads are actually worken and reevaluated
>> for scheduling. In the case of SCHED_OTHER, it means basically nothing,
>> considering the scheduling policy is arbitrary.
>>
> Clearly the point is that one of the waiting threads is waken and gets 
> the mutex, and it doesn't matter which thread is chosen. I.e., whatever 
> thread the scheduling policy chooses. The fact that SCHED_OTHER can 
> choose arbitrarily is immaterial, it still can only choose one of the 
> waiting threads.
> 

I don't know that it exactly says one of the waiting threads must get the
mutex.

> The fact that SCHED_OTHER's scheduling behavior is undefined is not free 
> license to implement whatever you want. Scheduling policies are an 
> optional feature; the basic thread behavior must still be consistent 
> even on systems that don't implement scheduling policies.
> 

It just so happens that normal tasks in Linux run in SCHED_OTHER. It
is irrelevant whether it might be an optional feature or not.

-- 
SUSE Labs, Novell Inc.
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