Re: futex(2) man page update help request

[Torvald Riegel <triegel@redhat.com>]

On Sat, 2015-01-24 at 11:05 +0100, Thomas Gleixner wrote:
> On Fri, 23 Jan 2015, Torvald Riegel wrote:
> 
> > On Fri, 2015-01-16 at 16:46 -0800, Darren Hart wrote:
> > > On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)"
> > > <mtk.manpages@gmail.com> wrote:
> > > 
> > > >Color me stupid, but I can't see this in futex_requeue(). Where is that
> > > >check that is "independent of the requeue type (normal/pi)"?
> > > >
> > > >When I look through futex_requeue(), all the likely looking sources
> > > >of EINVAL are governed by a check on the 'requeue_pi' argument.
> > > 
> > > 
> > > Right, in the non-PI case, I believe there are valid use cases: move to
> > > the back of the FIFO, for example (OK, maybe the only example?).
> > 
> > But we never guarantee a futex is a FIFO, or do we?  If we don't, then
> > such a requeue could be implemented as a no-op by the kernel, which
> > would sort of invalidate the use case.
> > 
> > (And I guess we don't want to guarantee FIFO behavior for futexes.)
> 
> The (current) behaviour is:
> 
>     real-time threads:   FIFO per priority level
>     sched-other threads: FIFO independent of nice level
> 
> The wakeup is priority ordered. Highest priority level first.

OK.

But, just to be clear, do I correctly understand that you do not want to
guarantee FIFO behavior in the specified futex semantics?  I think there
are cases where being able to *rely* on FIFO (now and on all future
kernels) would be helpful for users (e.g., on POSIX/C++11 condvars and I
assume in other ordered-wakeup cases too) -- but at the same time, this
would constrain future futex implementations.