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SubjectRe: Next round: revised futex(2) man page for review
On Tue, 28 Jul 2015, Darren Hart wrote:
> Found it on libc-alpha, here it is for reference:
>
> From: Rich Felker <dalias@libc.org>
> Date: Wed, 29 Oct 2014 22:43:17 -0400
> To: Darren Hart <dvhart@infradead.org>
> Cc: Carlos O'Donell <carlos@redhat.com>, Roland McGrath <roland@hack.frob.com>,
> Torvald Riegel <triegel@redhat.com>, GLIBC Devel <libc-alpha@sourceware.org>,
> Michael Kerrisk <mtk.manpages@gmail.com>
> Subject: Re: Add futex wrapper to glibc?
>
> On Wed, Oct 29, 2014 at 06:59:15PM -0700, Darren Hart wrote:
> > > We are IMO at the stage where futex is stable, few things are
> > > changing, and with documentation in place, I would consider adding a
> > > futex wrapper.
> >
> > Yes, at least for the defined OP codes. New OPs may be added of
> > course, but that isn't a concern for supporting what exists today, and
> > doesn't break compatibility.
> >
> > I wonder though... can we not wrap FUTEX_REQUEUE? It's fundamentally
> > broken. FUTEX_CMP_REQUEUE should *always* be used instead. The glibc
> > wrapper is one way to encourage developers to do the right thing
> > (don't expose the bad op in the header).
>
> You're mistaken here. There are plenty of valid ways to use
> FUTEX_REQUEUE - for example if the calling thread is requeuing the
> target(s) to a lock that the calling thread owns. Just because it
> doesn't meet the needs of the way glibc was using it internally
> doesn't mean it's useless for other applications.
>
> In any case, I don't think there's a proposal to intercept/modify the
> commands to futex, just to pass them through (and possibly do a
> cancellable syscall for some of them).

Fair enough. Did not think about the requeue to futex held by the
caller case. In that case FUTEX_REQUEUE works as advertised.

Thanks,

tglx


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