linux-kernel - RE: [PATCH] x86: fix and improve cmpxchg_double{,

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Message-Id: <4F043DEE020000780006A575@nat28.tlf.novell.com>
Date:	Wed, 04 Jan 2012 10:54:22 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Laight" <David.Laight@...LAB.COM>,
	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	"Christoph Lameter" <cl@...ux.com>, <linux-kernel@...r.kernel.org>,
	"netdev" <netdev@...r.kernel.org>, <hpa@...or.com>
Subject: RE: [PATCH] x86: fix and improve cmpxchg_double{,_local}()

 >>> On 04.01.12 at 11:36, "David Laight" <David.Laight@...LAB.COM> wrote:

>> BTW atomic64_set() asm() contraints are wrong :
>> 
>> static inline void atomic64_set(atomic64_t *v, long long i)
>> {
>>         unsigned high = (unsigned)(i >> 32);
>>         unsigned low = (unsigned)i;
>>         asm volatile(ATOMIC64_ALTERNATIVE(set)
>>                      : "+b" (low), "+c" (high)
>>                      : "S" (v)
>>                      : "eax", "edx", "memory"
>>                      );
>> }
>> 
>> 
>> ebx/ecx registers are not modified by cmpxchg8b (or the 
>> atomic64_set_386 emulation). Only eax/edx can be modified.

Same would be true for atomic64_xchg() and the use of "+c" (v)
in subsequent functions (whether unnecessarily strict or too lax
varies).

> Isn't it also possible to constrain the "memory"
> constraint to only apply to '*v' not all of memory?
> I can't remember the syntax off hand though...

Absolutely - "=m" (v->counter) would be the right way to specify
this.

Jan

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