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Subject: RE: Bitops source problem
Date: Thu, 17 Jan 2008 12:08:19 +0530
Message-ID: <1145971D427D7F43B0DBDAE22221C4B502CCE22D@nipns00a210.FPS.NIHILENT.COM>
Thread-Topic: Bitops source problem
Thread-Index: AchYzI8QM7KoW+7+QrWLm0hE2kWh1gABoAVA
From: "Pravin Nanaware" <p.nanaware@nihilent.com>
To: "John Hubbard" <john.hubbard@gmail.com>
Cc: "LKML" <linux-kernel@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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Lines: 72

Thanks for the reply John. 

Then, I think there is a problem with the function written below which is meant to be atomic.

static __inline__ void change_bit(int nr, volatile void * addr)
{
        __asm__ __volatile__(
                "btcl %1,%0"
                :"=m" (ADDR)
                :"Ir" (nr));
}

Regards,
Pravin


-----Original Message-----
From: John Hubbard [mailto:john.hubbard@gmail.com]
Sent: Thursday, January 17, 2008 11:17 AM
To: Pravin Nanaware
Cc: LKML
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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