Changes:
- membar_producer();
*p = v;
=>
atomic_store_release(p, v);
(Effectively like using membar_exit instead of membar_producer,
which is what we should have been doing all along so that stores by
the `reader' can't affect earlier loads by the writer, such as
KASSERT(p->refcnt == 0) in the writer and atomic_inc(&p->refcnt) in
the reader.)
- p = *pp;
if (p != NULL) membar_datadep_consumer();
=>
p = atomic_load_consume(pp);
(Only makes a difference on DEC Alpha. As long as lists generally
have at least one element, this is not likely to make a big
difference, and keeps the code simpler and clearer.)
No other functional change intended. While here, annotate each
synchronizing load and store with its counterpart in a comment.
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