Re: sinval synchronization considered harmful

On Tue, Jul 26, 2011 at 9:57 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Jul 26, 2011 at 4:38 PM, Noah Misch <noah@2ndquadrant.com> wrote:
>> No new ideas come to mind, here.
>
> OK, I have a new idea.  :-)
>
> 1. Add a new flag to each procState called something like "timeToPayAttention".
> 2. Each call to SIGetDataEntries() iterates over all the ProcStates
> whose index is < lastBackend and sets stateP->timeToPayAttention =
> TRUE for each.
> 3. At the beginning of SIGetDataEntries(), we do an unlocked if
> (!stateP->timeToPayAttention) return 0.
> 4. Immediately following that if statement and before acquiring any
> locks, we set stateP->timeToPayAttention = FALSE.
>
> The LWLockRelease() in SIGetDataEntries() will be sufficient to force
> all of the stateP->timeToPayAttention writes out to main memory, and
> the read side is OK because we either just took a lock (which acted as
> a fence) or else there's a race anyway.  But unlike my previous
> proposal, it doesn't involve *comparing* anything.  We needn't worry
> about whether we could read two different values that are through
> great misfortune out of sync because we're only reading one value.
>
> If, by chance, the value is set to true just after we set it to false,
> that won't mess us up either: we'll still read all the messages after
> acquiring the lock.

There turned out to be a little bit of further subtlety to this, but
it seems to work.  Patch attached.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company