linux-kernel - Re: [PATCH 10/11] x86, rwsem: provide __down_write

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Date:	Wed, 20 Apr 2016 16:58:26 -0400
From:	Michal Hocko <mhocko@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>, x86@...nel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	linux-arch@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 10/11] x86, rwsem: provide __down_write_killable

On Wed 20-04-16 22:45:01, Borislav Petkov wrote:
> On Wed, Apr 20, 2016 at 11:04:05AM -0700, H. Peter Anvin wrote:
> > The reason it breaks is because the same register can't be an
> > input-output register and a separate input. However, the input side of
> > the input-output is probably undefined, and so gcc may not notice.
> 
> So Michal and I talked about this a while ago. Why do we need the '"a"
> (sem)' input dependency if '"+a" (ret)' already supplies the same thing?
> 
> There's also that "=d" (tmp) thing which we don't really need as an
> output, right?
> 
> I.e., can we simplify like this?

I am for any simplification, my gcc-asm-foo is just too weak and I
wanted my change to be as minimal as possible. So if you feel you can
clean up this I would more than welcome that. Maybe a follow up patch
would be a better approach so that we can check that the generated code
hasn't changed.

Thanks!
-- 
Michal Hocko
SUSE Labs