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authorDavid Howells <dhowells@redhat.com>2006-06-25 05:48:49 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2006-06-25 10:01:19 -0700
commit2b94895b9281976cf2a0f2f89bf41ee219742235 (patch)
treebf42dba4bfb46c708d5fbb77ce7010f0058c3389 /Documentation/memory-barriers.txt
parent326f28e9ec4b2619c2fd410593fc95fcb0ba6b41 (diff)
downloadlinux-2b94895b9281976cf2a0f2f89bf41ee219742235.tar.gz
[PATCH] Another couple of alterations to the memory barrier doc
Make another couple of alterations to the memory barrier document following suggestions by Alan Stern and in co-operation with Paul McKenney: (*) Rework the point of introduction of memory barriers and the description of what they are to reiterate why they're needed. (*) Modify a statement about the use of data dependency barriers to note that other barriers can be used instead (as they imply DD-barriers). Signed-off-by: David Howells <dhowells@redhat.com> Acked-By: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'Documentation/memory-barriers.txt')
-rw-r--r--Documentation/memory-barriers.txt15
1 files changed, 10 insertions, 5 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 4710845dbac..cc53f47a83e 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -262,9 +262,14 @@ What is required is some way of intervening to instruct the compiler and the
CPU to restrict the order.
Memory barriers are such interventions. They impose a perceived partial
-ordering between the memory operations specified on either side of the barrier.
-They request that the sequence of memory events generated appears to other
-parts of the system as if the barrier is effective on that CPU.
+ordering over the memory operations on either side of the barrier.
+
+Such enforcement is important because the CPUs and other devices in a system
+can use a variety of tricks to improve performance - including reordering,
+deferral and combination of memory operations; speculative loads; speculative
+branch prediction and various types of caching. Memory barriers are used to
+override or suppress these tricks, allowing the code to sanely control the
+interaction of multiple CPUs and/or devices.
VARIETIES OF MEMORY BARRIER
@@ -461,8 +466,8 @@ Whilst this may seem like a failure of coherency or causality maintenance, it
isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
Alpha).
-To deal with this, a data dependency barrier must be inserted between the
-address load and the data load:
+To deal with this, a data dependency barrier or better must be inserted
+between the address load and the data load:
CPU 1 CPU 2
=============== ===============