Re: [PATCH] Document Linux's memory barriers [try #5]

Hello,
This is just a question about intended meaning in a paragraph
at the end of the "What are memory barriers?" section.

From: Paul E. McKenney Thu Mar 16 2006 - 18:14:11 EST
On Wed, Mar 15, 2006 at 02:23:19PM +0000, David Howells wrote:

The attached patch documents the Linux kernel's memory barriers.

I've updated it from the comments I've been given.

. . .

Good stuff!!! Please see comments interspersed, search for empty lines.

One particularly serious issue involve your smp_read_barrier_depends()
example.

Signed-Off-By: David Howells <dhowells@xxxxxxxxxx>
---
warthog>diffstat -p1 /tmp/mb.diff
Documentation/memory-barriers.txt | 1039
++++++++++++++++++++++++++++++++++++++
1 files changed, 1039 insertions(+)

diff --git a/Documentation/memory-barriers.txt
b/Documentation/memory-barriers.txt
new file mode 100644
index 0000000..fd7a6f1
--- /dev/null
+++ b/Documentation/memory-barriers.txt
@@ -0,0 +1,1039 @@
+ ============================
+ LINUX KERNEL MEMORY BARRIERS
+ ============================

. . .

+=========================
+WHAT ARE MEMORY BARRIERS?
+=========================
+

. . .

+It is also guaranteed that a CPU will be self-consistent: it will see its _own_
+accesses appear to be correctly ordered, without the need for a memory barrier.
+For instance with the following code:
+
+ X = *A;
+ *A = Y;
+ Z = *A;
+
+assuming no intervention by an external influence, it can be taken that:
+
+ (*) X will hold the old value of *A, and will never happen after the write and
+ thus end up being given the value that was assigned to *A from Y instead;

Seems like the subject of "will never happen" is the read from memory for the
asmt to X, but does that sentence say that?

+ and
+
+ (*) Z will always be given the value in *A that was assigned there from Y, and
+ will never happen before the write, and thus end up with the same value
+ that was in *A initially.

Similarly, the read from memory for the asmt to Z won't precede the write of Y
to *A.

+
+(This is ignoring the fact that the value initially in *A may appear to be the
+same as the value assigned to *A from Y).
+
+
+=================================
+WHERE ARE MEMORY BARRIERS NEEDED?
+=================================

It seems to require more effort than necessary to understand in regard to
all that is presented in this document.

Thanks.
Suzanne
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