linux-kernel - Re: [PATCH 1/23] document preferred use of volatile with atomic

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Message-ID: <20070813235423.GO13410@linux.vnet.ibm.com>
Date:	Mon, 13 Aug 2007 16:54:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org,
	Segher Boessenkool <segher@...nel.crashing.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	Chris Friesen <cfriesen@...tel.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>
Subject: Re: [PATCH 1/23] document preferred use of volatile with atomic_t

On Mon, Aug 13, 2007 at 07:04:15AM -0400, Chris Snook wrote:
> From: Chris Snook <csnook@...hat.com>
> 
> 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 <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Chris Snook <csnook@...hat.com>
> 
> --- 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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