Re: Next round: revised futex(2) man page for review

[Thomas Gleixner <tglx@linutronix.de>]

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