On Mon, Aug 13, 2007 at 07:04:15AM -0400, Chris Snook wrote:
> From: Chris Snook <[email protected]>
>
> Document proper use of volatile for atomic_t operations.
Looks good, as did a once-over on the arch-specific files. Good stuff!!!
Acked-by: Paul E. McKenney <[email protected]>
> Signed-off-by: Chris Snook <[email protected]>
>
> --- linux-2.6.23-rc3-orig/Documentation/atomic_ops.txt 2007-07-08 19:32:17.000000000 -0400
> +++ linux-2.6.23-rc3/Documentation/atomic_ops.txt 2007-08-13 03:36:43.000000000 -0400
> @@ -12,13 +12,20 @@
> C integer type will fail. Something like the following should
> suffice:
>
> - typedef struct { volatile int counter; } atomic_t;
> + typedef struct { int counter; } atomic_t;
> +
> + Historically, counter has been declared as a volatile int. This
> +is now discouraged in favor of explicitly casting it as volatile where
> +volatile behavior is required. Most architectures will only require such
> +a cast in atomic_read() and atomic_set(), as well as their 64-bit versions
> +if applicable, since the more complex atomic operations directly or
> +indirectly use assembly that results in volatile behavior.
>
> The first operations to implement for atomic_t's are the
> initializers and plain reads.
>
> #define ATOMIC_INIT(i) { (i) }
> - #define atomic_set(v, i) ((v)->counter = (i))
> + #define atomic_set(v, i) (*(volatile int *)&(v)->counter = (i))
>
> The first macro is used in definitions, such as:
>
> @@ -38,7 +45,7 @@
>
> Next, we have:
>
> - #define atomic_read(v) ((v)->counter)
> + #define atomic_read(v) (*(volatile int *)&(v)->counter)
>
> which simply reads the current value of the counter.
>
-
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