| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Latches and barriers |
| Date: | 2015-01-12 17:44:56 |
| Message-ID: | CA+TgmoYVcdCsPKW3hoE8bpeJk8+Gx2vFHuRQCdWLYqj7nCQmPA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Jan 12, 2015 at 11:27 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2015-01-12 11:03:42 -0500, Tom Lane wrote:
>> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
>> > While it might not be required for existing latch uses (I'm *not* sure
>> > that's true)
>
> I think at least syncrep.c might not be correct. In SyncRepWakeQueue()
> it sets PGPROC->syncRepState without the necessary barriers (via locks),
> although it does use them in SyncRepWaitForLSN().
>
> It is, perhaps surprisingly to many, not sufficient to take a spinlock,
> change the flag, release it and then set the latch - the release alone
> doesn't guarantee a sufficient barrier unless looking at the flag is
> also protected by the spinlock.
I thought we decided that a spinlock acquire or release should be a
full barrier.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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