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

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Date:	Mon, 13 Aug 2007 07:04:15 -0400
From:	Chris Snook <csnook@...hat.com>
To:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	akpm@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	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: [PATCH 1/23] document preferred use of volatile with atomic_t

From: Chris Snook <csnook@...hat.com>

Document proper use of volatile for atomic_t operations.

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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