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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