Memory Barriers with STM and MVar

Andrew Martin andrew.thaddeus at
Sat Jun 1 14:11:20 UTC 2019

What I'm doing is using STM (but it could be done with MVar instead) to
guard access to a rotating queue that is a large mmapped region of memory
(in my situation, it also happens to be backed by a file on disk). There
can only be one writer at a time. There can be many readers, but the
readers cannot read from the head of the queue if the writer is still
working on it. They must wait until the writer is finished. So, every
thread has the same base address and then they are all working at different
offsets into the queue. But when the writer tries to tell the readers "I'm
done writing to this area", how can I be sure that the readers will
actually see the writes to memory? That's the difficulty I'm having.

On Sat, Jun 1, 2019 at 9:31 AM Elliot Cameron <eacameron at> wrote:

> For my education, can you describe your use case a bit more? Using MVar as
> you did in your example seems like a strange use case. Normally you'd just
> put the pointer to the structure in the MVar itself.

-Andrew Thaddeus Martin
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