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Subject: Re: set_bit() is broken on i386?
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Chuck Ebbert <76306.1226@compuserve.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>, Andi Kleen <ak@suse.de>,
       Andrew Morton <akpm@osdl.org>, Ingo Molnar <mingo@redhat.com>,
       Linus Torvalds <torvalds@osdl.org>
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Date: Fri, 20 Jan 2006 20:15:07 -0500
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On Fri, 2006-01-20 at 19:53 -0500, Chuck Ebbert wrote:

> #define ADDR (*(volatile long *) addr)
> static inline void set_bit(int nr, volatile unsigned long * addr)
> {
> 	__asm__ __volatile__( "lock ; "
> 		"btsl %1,%0"
> 		:"=m" (ADDR)
> 		:"Ir" (nr));
> }

The asm needs a memory clobber in order to avoid reordering with the
assignment to b[1]:

static inline void set_bit(int nr, volatile unsigned long * addr)
{
	__asm__ __volatile__( "lock ; "
		"btsl %1,%0"
		:"=m" (ADDR)
		:"Ir" (nr)
		: "memory");
}

This works consistently correctly for me.

Cheers,
  Trond