Subj : Re: Memory visibility and MS Interlocked instructions
To   : comp.programming.threads
From : Scott Meyers
Date : Thu Aug 25 2005 02:04 pm

Alexander Terekhov wrote:
 > No on the first part to the extent that "... thread W (the writer)
 > writes a value to variable x and thread R (the reader) later reads
 > the value of x ...  to guarantee that R sees the value written by
 > W, W must follow the write with a release membar and R must
 > precede the read with an acquire membar" is not true on shared
 > memory MP hardware level where acquire and release merely ensure
 > ordering with respect to other accesses.

I was actually asking about the truth of this statement:

    Both writer and reader must participate in a handshake to ensure that
    writer changes to memory are visible to readers in a relaxed memory
    architecture.

What struck me most about the documentation of the interlocked instructions
was that it suggested there was no need for a handshake, i.e., if the
writer called an interlocked routine, there was no need for a reader to do
anything special when reading.  From the responses I've seen in this thread,
I get the impression that a handshake *is* required, that both writer and
reader must do something to ensure that the reader sees what the writer
last wrote, at least in the general case.

As for your specific statement above, I gather you mean that on some
architectures, more specific membars may be necessary rather than just
release by the writer and acquire by the reader.  Is that correct?

Scott

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