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Message-ID: <20070813110415.GA24018@shell.boston.redhat.com>
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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