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

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Date:	Wed, 20 Apr 2016 15:29:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Michal Hocko <mhocko@...nel.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 April 20, 2016 2:36:37 PM PDT, Borislav Petkov <bp@...en8.de> wrote:
>On Wed, Apr 20, 2016 at 02:06:33PM -0700, H. Peter Anvin wrote:
>> Setting ret to sem doesn't make any sense.  Just use "=a" and "a".
>
>Yeah, that's what Michal's patch ontop does.
>
>And to answer my own question: we need the "a" (sem) input for the fast
>path.
>
>I guess we can still move "1" (RWSEM_ACTIVE_WRITE_BIAS) before the
>asm():
>
>	long tmp = RWSEM_ACTIVE_WRITE_BIAS;
>
>One thing I'm still not clear on is why we need the output tmp operand:
>"=d" (tmp) ?

Since it is a fixed register we could just mark edx clobbered, but with more flexible register constraints it can permit gcc to allocate a temp resister for us.
-- 
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