Re: [PATCH] Document non-semantics of atomic_read() and atomic_set()
Acked-by: Christoph Lameter <[EMAIL PROTECTED]> - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH] Document non-semantics of atomic_read() and atomic_set()
On Mon, Sep 10, 2007 at 07:19:44PM -0400, Chris Snook wrote:
> From: Chris Snook <[EMAIL PROTECTED]>
>
> Unambiguously document the fact that atomic_read() and atomic_set()
> do not imply any ordering or memory access, and that callers are
> obligated to explicitly invoke barriers as needed to ensure that
> changes to atomic variables are visible in all contexts that need
> to see them.
Acked-by: Paul E. McKenney <[EMAIL PROTECTED]>
> Signed-off-by: Chris Snook <[EMAIL PROTECTED]>
>
> --- a/Documentation/atomic_ops.txt2007-07-08 19:32:17.0 -0400
> +++ b/Documentation/atomic_ops.txt2007-09-10 19:02:50.0 -0400
> @@ -12,7 +12,11 @@
> 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 volatile. This is now
> +discouraged. See Documentation/volatile-considered-harmful.txt for the
> +complete rationale.
>
> The first operations to implement for atomic_t's are the
> initializers and plain reads.
> @@ -42,6 +46,22 @@
>
> which simply reads the current value of the counter.
>
> +*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***
> +
> +Some architectures may choose to use the volatile keyword, barriers, or
> +inline assembly to guarantee some degree of immediacy for atomic_read()
> +and atomic_set(). This is not uniformly guaranteed, and may change in
> +the future, so all users of atomic_t should treat atomic_read() and
> +atomic_set() as simple C assignment statements that may be reordered or
> +optimized away entirely by the compiler or processor, and explicitly
> +invoke the appropriate compiler and/or memory barrier for each use case.
> +Failure to do so will result in code that may suddenly break when used with
> +different architectures or compiler optimizations, or even changes in
> +unrelated code which changes how the compiler optimizes the section
> +accessing atomic_t variables.
> +
> +*** YOU HAVE BEEN WARNED! ***
> +
> Now, we move onto the actual atomic operation interfaces.
>
> void atomic_add(int i, atomic_t *v);
-
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[PATCH] Document non-semantics of atomic_read() and atomic_set()
From: Chris Snook <[EMAIL PROTECTED]>
Unambiguously document the fact that atomic_read() and atomic_set()
do not imply any ordering or memory access, and that callers are
obligated to explicitly invoke barriers as needed to ensure that
changes to atomic variables are visible in all contexts that need
to see them.
Signed-off-by: Chris Snook <[EMAIL PROTECTED]>
--- a/Documentation/atomic_ops.txt 2007-07-08 19:32:17.0 -0400
+++ b/Documentation/atomic_ops.txt 2007-09-10 19:02:50.0 -0400
@@ -12,7 +12,11 @@
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 volatile. This is now
+discouraged. See Documentation/volatile-considered-harmful.txt for the
+complete rationale.
The first operations to implement for atomic_t's are the
initializers and plain reads.
@@ -42,6 +46,22 @@
which simply reads the current value of the counter.
+*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***
+
+Some architectures may choose to use the volatile keyword, barriers, or
+inline assembly to guarantee some degree of immediacy for atomic_read()
+and atomic_set(). This is not uniformly guaranteed, and may change in
+the future, so all users of atomic_t should treat atomic_read() and
+atomic_set() as simple C assignment statements that may be reordered or
+optimized away entirely by the compiler or processor, and explicitly
+invoke the appropriate compiler and/or memory barrier for each use case.
+Failure to do so will result in code that may suddenly break when used with
+different architectures or compiler optimizations, or even changes in
+unrelated code which changes how the compiler optimizes the section
+accessing atomic_t variables.
+
+*** YOU HAVE BEEN WARNED! ***
+
Now, we move onto the actual atomic operation interfaces.
void atomic_add(int i, atomic_t *v);
-
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