Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 9 Aug 2007 07:32:55 -0700

On Thu, Aug 09, 2007 at 09:24:42AM -0400, Chris Snook wrote:
> From: Chris Snook <[email-masked]>
> 
> Purify volatile use for atomic[64]_t on alpha.

Why not the same access-once semantics for atomic_set() as
for atomic_read()?  As this patch stands, it might introduce
architecture-specific compiler-induced bugs due to the fact that
atomic_set() used to imply volatile behavior but no longer does.

See below for one approach to fixing this.

> Signed-off-by: Chris Snook <[email-masked]>
> 
> --- linux-2.6.23-rc2-orig/include/asm-alpha/atomic.h	2007-07-08 19:32:17.000000000 -0400
> +++ linux-2.6.23-rc2/include/asm-alpha/atomic.h	2007-08-09 09:19:00.000000000 -0400
> @@ -14,18 +14,22 @@
> 
> 
>  /*
> - * Counter is volatile to make sure gcc doesn't try to be clever
> - * and move things around on us. We need to use _exactly_ the address
> - * the user gave us, not some alias that contains the same information.
> + * Make sure gcc doesn't try to be clever and move things around
> + * on us. We need to use _exactly_ the address the user gave us,
> + * not some alias that contains the same information.
>   */
> -typedef struct { volatile int counter; } atomic_t;
> -typedef struct { volatile long counter; } atomic64_t;
> +typedef struct { int counter; } atomic_t;
> +typedef struct { long counter; } atomic64_t;
> 
>  #define ATOMIC_INIT(i)		( (atomic_t) { (i) } )
>  #define ATOMIC64_INIT(i)	( (atomic64_t) { (i) } )
> 
> -#define atomic_read(v)		((v)->counter + 0)
> -#define atomic64_read(v)	((v)->counter + 0)
> +/*
> + * Casting to volatile here minimizes the need for barriers,
> + * without having to declare the type itself as volatile.
> + */
> +#define atomic_read(v)		(*(volatile int *)&(v)->counter + 0)
> +#define atomic64_read(v)	(*(volatile long *)&(v)->counter + 0)
> 
>  #define atomic_set(v,i)		((v)->counter = (i))
>  #define atomic64_set(v,i)	((v)->counter = (i))

How about:

#define atomic_set(v,i)		((*(volatile int *)&(v)->counter) = (i))
#define atomic64_set(v,i)	((*(volatile long *)&(v)->counter) = (i))

						Thanx, Paul

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