[Bug nptl/13165] pthread_cond_wait() can consume a signal that was sent before it started waiting

["triegel at redhat dot com" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13165

--- Comment #23 from Torvald Riegel <triegel at redhat dot com> 2012-09-20 11:58:26 UTC ---
(In reply to comment #21)
> (In reply to comment #20)
> > The standard indeed doesn't talk about the "future".  It doesn't make a sort of
> > lower-bound requirement on which threads have to be considered blocked, but no
> > upper bound.  If you think there's an upper bound, please point the requirement
> > in the standard.  If there is no required upper bound, it's up to the
> > implementation how to deal with that.
> 
> "The pthread_cond_broadcast() and pthread_cond_signal() functions shall have no
> effect if there are no threads currently blocked on cond."
> 
> How about this as an upper bound?

This states something in relation to those threads that are considered to be
blocked.  It does not state anything about which threads can or have to be
considered to be blocked.  So, it can't be an upper bound.

> If implementations are allowed to determine
> the set of blocked threads at any point in time they see fit, there would be no
> way to define "currently blocked" at all and this sentence couldn't make any
> sense.

There is a lower bound (or minimum requirement) based on the happens-before via
the mutex (hence "currently").  The sentence allows the implementation to let
the signal have no effect if there is no thread that has to be considered
blocked with the assumption of the lower bound.  Assuming more threads to be
blocked is the same as allowing spurious wake-ups.

> And also:
> 
> ".... however, if predictable scheduling behavior is required, then that mutex
> shall be locked by the thread calling pthread_cond_broadcast() or
> pthread_cond_signal()."
> 
> If I accept your argument, there will be no way to determine at least a set of
> threads from which the woken thread will be chosen, so why does the standard
> talk about predictability?

There is the lower bound, which does determine properties of this set.

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