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[PATCH] Another couple of alterations to the memory barrier doc

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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>
This commit is contained in:
David Howells 2006-06-25 05:48:49 -07:00 committed by Linus Torvalds
parent 326f28e9ec
commit 2b94895b92
1 changed files with 10 additions and 5 deletions
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15
Documentation/memory-barriers.txt
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@ -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
=============== ===============