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

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Message-ID: <AANLkTinmxcjRUj-JRtmO3JSAGWoiW+ec7JzTBpL32C=t@mail.gmail.com>
Date:	Sun, 13 Mar 2011 17:55:45 -0700
From:	Darren Hart <darren@...art.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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()

I have to apologize. These landed right as I was leaving for OSTS and
I didn't have time to review them properly.  I wondered about the
preempt vs pagefault disable and wondered how exit_robust_list() was
covered in Michel's comments (perhaps somewhere higher up the call
chain). I leaned on the review of others when I should have raised the
questions even if I didn't have the time to dig into them myself.
Linus shouldn't have had to raise those questions, I'll do better at
this in the future.

I noticed that my name is the only one in futex.c with an email
address in the header - that email address is no longer valid, and it
delayed the patches getting to my inbox - I will submit a patch to fix
that. I should catch them sooner now, regardless, with improved LKML
filters.

Thanks,

Darren Hart


On Sun, Mar 13, 2011 at 3:49 PM, Linus Torvalds
<torvalds@...ux-foundation.org> 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.
>
>                              Linus
>



-- 
Darren Hart
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