linux-kernel - Re: process 'stuck' at exit.

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Message-ID: <alpine.DEB.2.02.1312110025340.28330@ionos.tec.linutronix.de>
Date:	Wed, 11 Dec 2013 00:28:29 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: process 'stuck' at exit.

On Tue, 10 Dec 2013, Linus Torvalds wrote:

> On Tue, Dec 10, 2013 at 2:57 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > But how does the access_ok() move do anything helpful here?
> 
> Just making it all more obvious.
> 
> > We really need it for the fastpath !fshared case, but for the fshared
> > case you actively break working code, because you force a VERIFY_WRITE
> > check into it. The VERIFY_WRITE is necessary for !fshared, because
> > there is no way that one thread can map the futex RO and the other RW,
> > right?
> 
> Nobody actually uses that argument any more (it goes back to the old
> i386 "let's manually verify that we have write permissions, because
> the CPU doesn't do it for us in the trap handling"), and it should
> probably be removed.

Fair enough.
 
> But you're right that it's at least misleading. I'd love to remove it
> entirely, because it's not even syntax-checked, and it's confusing.
> But that would be a humongous patch.

Well, we should ask Julia for a coccinelle patch to limit the
wreckage. :)

Seriously, if that VERIFY_WRITE is completely useless we really want
to get rid of it.
 
> So these days, "access_ok()" literally just checks that the address is
> in the user address space range. And that would seem to always be
> appropriate for futexes, so why not just do it in the generic code?

Agreed, but as long as the VERIFY_WRITE argument is there it needs at
least a big fat comment :)

Thanks,

	tglx
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