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| author | David Howells <dhowells@redhat.com> | 2006-06-25 05:48:49 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@g5.osdl.org> | 2006-06-25 10:01:19 -0700 |
| commit | 2b94895b9281976cf2a0f2f89bf41ee219742235 (patch) | |
| tree | bf42dba4bfb46c708d5fbb77ce7010f0058c3389 /Documentation | |
| parent | 326f28e9ec4b2619c2fd410593fc95fcb0ba6b41 (diff) | |
| download | blackbird-op-linux-2b94895b9281976cf2a0f2f89bf41ee219742235.tar.gz blackbird-op-linux-2b94895b9281976cf2a0f2f89bf41ee219742235.zip | |
[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')
| -rw-r--r-- | Documentation/memory-barriers.txt | 15 |
1 files changed, 10 insertions, 5 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 4710845dbac4..cc53f47a83e8 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 =============== =============== |
