From: Chris Snook <[email protected]>
Document proper use of volatile for atomic_t operations.
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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