On Thu, Aug 09, 2007 at 09:24:42AM -0400, Chris Snook wrote:
> From: Chris Snook <[EMAIL PROTECTED]>
>
> 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 PROTECTED]>
>
> --- 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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