On Thu, 2010-09-16 at 12:55 +0100, David Howells wrote:
> If there's a lock+unlock between, then this counts as a full memory
> barrier
I though it was unlock+lock.
>From memory-barriers.txt:
And a couple of implicit varieties:
(5) LOCK operations.
This acts as a one-way permeable barrier. It guarantees that all memory
operations after the LOCK operation will appear to happen after the LOCK
operation with respect to the other components of the system.
Memory operations that occur before a LOCK operation may appear to happen
after it completes.
A LOCK operation should almost always be paired with an UNLOCK operation.
(6) UNLOCK operations.
This also acts as a one-way permeable barrier. It guarantees that all
memory operations before the UNLOCK operation will appear to happen before
the UNLOCK operation with respect to the other components of the system.
Memory operations that occur after an UNLOCK operation may appear to
happen before it completes.
LOCK and UNLOCK operations are guaranteed to appear with respect to each
other strictly in the order specified.
The use of LOCK and UNLOCK operations generally precludes the need for
other sorts of memory barrier (but note the exceptions mentioned in the
subsection "MMIO write barrier").
Therefore:
A = 5
LOCK
UNLOCK
B = 6
May be observed as:
LOCK
B = 6
A = 5
UNLOCK
The sequence:
A = 5
UNLOCK
LOCK
B = 6
Is fully ordered.
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