linux-kernel - Re: [PATCH 1/3] futex: do not pagefault_disable in futex_atomic_cmpxchg

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Date:	Mon, 14 Mar 2011 10:13:30 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Michel Lespinasse <walken@...gle.com>,
	Darren Hart <darren@...art.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Matt Turner <mattst88@...il.com>,
	Russell King <linux@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Michal Simek <monstr@...str.eu>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"David S. Miller" <davem@...emloft.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] futex: do not pagefault_disable in
 futex_atomic_cmpxchg_inatomic()

On Sun, 13 Mar 2011, Linus Torvalds wrote:
> On Thu, Mar 10, 2011 at 6:47 PM, Michel Lespinasse <walken@...gle.com> wrote:
> > kernel/futex.c disables page faults before calling
> > futex_atomic_cmpxchg_inatomic(), so there is no need to do it again
> > within that function.
> 
> This seems totally bogus.
> 
> Even the comment is crap.
> 
> Sure, the callers may disable preemption, but that has NOTHING to do
> with "pagefault_disable()". Th epagefault_[en/dis]able functions will
> touch the preempt count EVEN IF PREEMPTION ISN'T EVEN ENABLED!
> 
> So what the f*ck does that "Note that preemption is disabled.." crap even mean?
> 
> The thing is made even worse by the fact that as far as I can tell,
> the comment simply isn't true at all (even if you were to ignore the
> fundamental confusion about preemption vs the pagefault
> disable/enable). Not all callers of futex_atomic_cmpxchg_inatomic() do
> anything of the sort, whether it's preemptibility _or_ the proper
> pagefault_disable/enable(). Just look at the exit_robust_list() ->
> handle_futex_death(), for example.
> 
> This kind of patch is the kind that personally makes me want to put
> you on a spam-list. Misleading commit messages with bogus and
> fundamentally incorrect added comments in the code. WTF?
> 
> Did I miss some patch that changed that, or is this really as horribly
> bad as I think it is? I see it already made it into -tip.

That's my fault. 

I really checked the call sites of futex_atomic_cmpxchg_inatomic() and
totally failed to see the one in handle_futex_death() which does not
use the helper function cmpxchg_futex_value_locked(). That helper
function is safe and does the right thing:

	pagefault_disable();
	curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
	pagefault_enable();

So, that means we have all call sites covered except one, which needs
to be fixed _AND_ also pushed into stable as all arch implementations
except ARM rely on the caller doing the pagefault_disable().

And I missed the bogus comment as well. Sigh.

Thanks,

	tglx
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