linux-kernel - Re: Bitops source problem

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Message-ID: <478EEBE4.40400@gmail.com>
Date:	Wed, 16 Jan 2008 21:47:16 -0800
From:	John Hubbard <john.hubbard@...il.com>
To:	Pravin Nanaware <p.nanaware@...ilent.com>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Bitops source problem

Pravin Nanaware wrote:
> Hi,
> 
> I was just going through the include file in the /usr/include/asm/bitops.h
> 
> The function description describes it as non-atomic but it seems it is not. 
> 
> static __inline__ void __change_bit(int nr, volatile void * addr)
> {
>         __asm__ __volatile__(
>                 "btcl %1,%0"
>                 :"=m" (ADDR)
>                 :"Ir" (nr));
> }
> 
> The kernel version I am using is 2.6.9-42. Is it right or am I missing something ?  
> 
> Thanks,
> Pravin
> 

The bitops.h comments are correct: the btc IA-32 instruction is only 
atomic if used with the lock prefix. The function above does not use the 
lock prefix, so it is not atomic.

thanks,
John Hubbard

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