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Subject: Re: futex(2) man page update help request
From: Torvald Riegel <triegel@redhat.com>
To: Darren Hart <dvhart@linux.intel.com>
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Date: Fri, 23 Jan 2015 19:19:05 +0100
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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.)