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

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Message-ID: <20160420204501.GA6815@pd.tnic>
Date:	Wed, 20 Apr 2016 22:45:01 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
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 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?

---
#define ____down_write(sem, slow_path)                  \
({                                                      \
        long tmp = RWSEM_ACTIVE_WRITE_BIAS;             \
	struct rw_semaphore* ret = sem;			\
                                                        \
        asm volatile("# beginning down_write\n\t"       \
                     LOCK_PREFIX "  xadd      %[tmp],(%[ret])\n\t"      \
                     /* adds 0xffff0001, returns the old value */ \
                     "  test " __ASM_SEL(%w[tmp],%k[tmp]) "," __ASM_SEL(%w[tmp],%k[tmp]) "\n\t" \
                     /* was the active mask 0 before? */\
                     "  jz        1f\n"                 \
                     "  call " slow_path "\n"           \
                     "1:\n"                             \
                     "# ending down_write"              \
                     : "+m" (sem->count), [ret] "+a" (ret) \
                     : [tmp] "d" (tmp)                  \
                     : "memory", "cc");                 \
        ret;                                            \
})

-- 
Regards/Gruss,
    Boris.

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