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

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Date:	Sun, 13 Mar 2011 15:49:15 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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 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.

                              Linus
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