Re: Latches and barriers

On Mon, Jan 12, 2015 at 11:27 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2015-01-12 11:03:42 -0500, Tom Lane wrote:
>> Andres Freund <andres@2ndquadrant.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