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

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Date:	Sun, 17 Apr 2016 12:59:21 -0400
From:	Michal Hocko <mhocko@...nel.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"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 13-04-16 14:49:43, Michal Hocko wrote:
> On Wed 13-04-16 12:27:31, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > I'm testing your patches today, if they are otherwise OK [...]
> > 
> > got this build failure:
> > 
> >   ./arch/x86/include/asm/rwsem.h:106:2: error: ‘asm’ operand has impossible constraints
> 
> Hmm, I have no idea why 64b didn't have problem with the asm but 32b
> complains. Anyway, the following makes both happy. I have checked the
> generated code for 64b and it hasn't changed after the patch. 32b also
> seems to be generating a proper code. My gcc asm()-foo is rather weak so
> I would feel better if somebody double checked after me.

Peter, Ingo, does the patch makes sense to you?
Thanks!

> ---
> From d23f4e6994670bf2c5d864f2190f21022b4499b2 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.com>
> Date: Wed, 13 Apr 2016 14:21:25 +0200
> Subject: [PATCH] x86: __down_read_trylock fix 32b build failure
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Ingo has noticed the following compilation error with CONFIG_X86_32=y
> ./arch/x86/include/asm/rwsem.h:106:2: error: ‘asm’ operand has impossible constraints
> 
> The reason seems to be that 32b doesn't like ret being input and output
> argument sharing the same register with sem which is only the input. Fix
> this by making ret output only and use %3 (aka sem) for xadd.
> 
> ret initialization is not needed now because this is done implicitly
> by the asm even for the fast path as both sem and ret share the same
> register.
> 
> Reported-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  arch/x86/include/asm/rwsem.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
> index d759c5f70f49..453744c1d347 100644
> --- a/arch/x86/include/asm/rwsem.h
> +++ b/arch/x86/include/asm/rwsem.h
> @@ -102,9 +102,9 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
>  #define ____down_write(sem, slow_path)			\
>  ({							\
>  	long tmp;					\
> -	struct rw_semaphore* ret = sem;			\
> +	struct rw_semaphore* ret;			\
>  	asm volatile("# beginning down_write\n\t"	\
> -		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"	\
> +		     LOCK_PREFIX "  xadd      %1,(%3)\n\t"	\
>  		     /* adds 0xffff0001, returns the old value */ \
>  		     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
>  		     /* was the active mask 0 before? */\
> @@ -112,7 +112,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
>  		     "  call " slow_path "\n"		\
>  		     "1:\n"				\
>  		     "# ending down_write"		\
> -		     : "+m" (sem->count), "=d" (tmp), "+a" (ret)	\
> +		     : "+m" (sem->count), "=d" (tmp), "=a" (ret)	\
>  		     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
>  		     : "memory", "cc");			\
>  	ret;						\
> -- 
> 2.8.0.rc3
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs