linux-kernel - Re: [x86] BUG: unable to handle kernel paging request at 00740060

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Date:	Tue, 8 Oct 2013 12:05:32 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Richard Henderson <rth@...ddle.net>,
	Jakub Jelinek <jakub@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [x86] BUG: unable to handle kernel paging request at 00740060

On Tue, Oct 8, 2013 at 11:51 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> I do not really understand inline assembly constraints, but I'll ask
> anyway.
>
>         +#define __GEN_RMWcc(fullop, var, cc, ...) \
>         +do { \
>         + asm volatile goto (fullop "; j" cc " %l[cc_label]" \
>         + : : "m" (var), ## __VA_ARGS__ \
>               ^^^^^^^^^
>
> don't we need
>
>         "+m" (var)

We have a memory clobber instead. So the memory is marked as input and
clobbered.

And we'd love to mark it "+m", but "ask goto" cannot have outputs.

For the serializing ones, the memory clobber is ok - they have barrier
semantics anyway. But we'd actually *want* to use "asm goto" for some
cases where the memory clobber is too big of a hammer, so if we ever
get input/output constraints to "asm goto" we'll be happy.

Of course, right now it looks like we shouldn't be in a rush to use
"asm goto" at all...

                   Linus
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