From: Chris Snook <[email protected]>
Update atomic_ops.txt to reflect the newly consistent behavior of
atomic_read(), and to note that volatile (in declarations) is now
considered harmful.
Signed-off-by: Chris Snook <[email protected]>
--- linux-2.6.23-rc2-orig/Documentation/atomic_ops.txt 2007-07-08 19:32:17.000000000 -0400
+++ linux-2.6.23-rc2/Documentation/atomic_ops.txt 2007-08-09 08:24:32.000000000 -0400
@@ -12,7 +12,7 @@
C integer type will fail. Something like the following should
suffice:
- typedef struct { volatile int counter; } atomic_t;
+ typedef struct { int counter; } atomic_t;
The first operations to implement for atomic_t's are the
initializers and plain reads.
@@ -38,9 +38,17 @@
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.
+which reads the counter as though it were volatile. This prevents the
+compiler from optimizing away repeated atomic_read() invocations without
+requiring a more expensive barrier(). Historically this has been
+accomplished by declaring the counter itself to be volatile, but the
+ambiguity of the C standard on the semantics of volatile make this practice
+vulnerable to overly creative interpretation by compilers. Explicit
+casting in atomic_read() ensures consistent behavior across architectures
+and compilers. Even with this convenience in atomic_read(), busy-waiters
+should call cpu_relax().
Now, we move onto the actual atomic operation interfaces.
-
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