On Tue, Jul 26, 2011 at 9:57 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Tue, Jul 26, 2011 at 4:38 PM, Noah Misch <n...@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
sinval-hasmessages.patch
Description: Binary data
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