On Mon, Jun 5, 2017 at 12:19 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > While I'm looking at this ... seems like there's a pretty basic coding- > rule violation here, namely that shm_toc_lookup() thinks it can read > toc->toc_nentry without any sort of locking. Since that field is declared > Size, this amounts to an assumption that 64-bit reads are atomic, which > is not per project practice. > > In practice it probably can't fail even if 64-bit reads aren't atomic, > simply because we'll never have enough entries in a shm_toc to make the > high-order half ever change. But that just begs the question why the > field is declared Size rather than int. I think we should make it the > latter.
Yeah. I think a shm_toc with more than 2^10 entries would probably perform badly enough that somebody would rewrite this entire module, so we don't really need to worry about having more than 2^31. Changing to int (or uint32) seems fine. > I am also thinking that most of the shm_toc functions need to have the > toc pointers declared as "volatile *", but particularly shm_toc_lookup. > That read_barrier call might prevent the hardware from reordering > accesses, but I don't think it stops the compiler from doing so. If it doesn't prevent both the hardware and the compiler from reordering, it's broken. See the comments for pg_read_barrier() in atomics.h. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
