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Message-ID: <20150826062958.GB50910@vmdeb7>
Date: Tue, 25 Aug 2015 23:29:58 -0700
From: Darren Hart <dvhart@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
Torvald Riegel <triegel@...hat.com>,
Carlos O'Donell <carlos@...hat.com>,
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linux-man <linux-man@...r.kernel.org>,
lkml <linux-kernel@...r.kernel.org>,
Davidlohr Bueso <dave@...olabs.net>,
Arnd Bergmann <arnd@...db.de>,
Steven Rostedt <rostedt@...dmis.org>,
Peter Zijlstra <peterz@...radead.org>,
Linux API <linux-api@...r.kernel.org>,
Roland McGrath <roland@...k.frob.com>,
Anton Blanchard <anton@...ba.org>,
Eric Dumazet <edumazet@...gle.com>,
bill o gallmeister <bgallmeister@...il.com>,
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Daniel Wagner <wagi@...om.org>, Rich Felker <dalias@...c.org>,
Andy Lutomirski <luto@...capital.net>,
bert hubert <bert.hubert@...herlabs.nl>,
Rusty Russell <rusty@...tcorp.com.au>,
Heinrich Schuchardt <xypron.glpk@....de>
Subject: Re: Next round: revised futex(2) man page for review
On Thu, Aug 20, 2015 at 01:17:03AM +0200, Thomas Gleixner wrote:
...
> > >> .\" FIXME XXX In discussing errors for FUTEX_CMP_REQUEUE_PI, Darren Hart
> > >> .\" made the observation that "EINVAL is returned if the non-pi
> > >> .\" to pi or op pairing semantics are violated."
> > >> .\" Probably there needs to be a general statement about this
> > >> .\" requirement, probably located at about this point in the page.
> > >> .\" Darren (or someone else), care to take a shot at this?
> > >
> > > Well, that's hard to describe because the kernel only has a limited
> > > way of detecting such mismatches. It only can detect it when there are
> > > non PI waiters on a futex and a PI function is called or vice versa.
> >
> > Hmmm. Okay, I filed your comments away for reference, but
> > hopefully someone can help with some actual text.
>
> I let Darren come up with something sensible :)
Heh, right, no pressure then...
I responded to Michael on this recently, copied here for reference:
FUTEX_WAIT_REQUEUE_PI can return -EINVAL if called with invalid parameters, such
as uaddr==uaddr2, or (in the case of SHARED futexes), the associated keys match
(meaning it's the same futex word - shared memory, inode, etc.). This can't
happen if the stated policy of requeueing from non-pi to pi is followed as the
same word cannot be both non-pi and pi at the same time, requiring them to be
unique futex words.
FUTEX_CMP_REQUEUE_PI will fail similarly if uaddr and uaddr2 are the same futex
word. Also, if nr_wake != 1.
But, to the point I was making above, FUTEX_CMP_REQUEUE_PI must requeue uaddr to
the same uaddr2 specified in the previous FUTEX_WAIT_REQUEUE_PI call.
FUTEX_WAIT_REQUEUE_PI sets up the operation, FUTEX_CMP_REQUEUE_PI completes it,
and they must agree on uaddr and uaddr2.
Michael, are you still looking for something more from me, or is this FIXME now
complete?
--
Darren Hart
Intel Open Source Technology Center
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