linux-kernel - Re: in x86 architecture ,why the function atomic_sub_and

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Message-ID: <4BBB9560.6070905@zytor.com>
Date:	Tue, 06 Apr 2010 13:11:12 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	杨硕 <shanqn@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: in x86 architecture ,why the function atomic_sub_and_test() does
 not  disable the interrupt?

On 04/05/2010 11:07 PM, Roland Dreier wrote:
>  > static inline int atomic_sub_and_test(int i, atomic_t *v)
>  > {
>  > 	unsigned char c;
>  > 
>  > 	asm volatile(LOCK_PREFIX "subl %2,%0; sete %1"
>  > 		     : "+m" (v->counter), "=qm" (c)
>  > 		     : "ir" (i) : "memory");
>  > 	return c;
>  > }
> 
> Why would disabling interrupts be necessary?  The LOCK_PREFIX makes the
> subl atomic, and the sete just operates using the flag set by subl, so
> it doesn't matter if any interrupts occur or not (since returning from
> an interrupt must obviously restore flags).

Even without the LOCK prefix, subl would be atomic against local
interrupts.  The LOCK prefix is only necessary to make it atomic against
other processors.

	-hpa
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