Re: SyncRepLock acquired exclusively in default configuration

On 2020/04/08 3:01, Ashwin Agrawal wrote:
>
> On Mon, Apr 6, 2020 at 2:14 PM Andres Freund <andres(at)anarazel(dot)de <mailto:andres(at)anarazel(dot)de>> wrote:
>
> > How about we change it to this ?
>
> Hm. Better. But I think it might need at least a compiler barrier /
> volatile memory load?  Unlikely here, but otherwise the compiler could
> theoretically just stash the variable somewhere locally (it's not likely
> to be a problem because it'd not be long ago that we acquired an lwlock,
> which is a full barrier).
>
>
> That's the part, I am not fully sure about. But reading the comment above SyncRepUpdateSyncStandbysDefined(), it seems fine.
>
> > Bring back the check which existed based on GUC but instead of just blindly
> > returning based on just GUC not being set, check
> > WalSndCtl->sync_standbys_defined. Thoughts?
>
> Hm. Is there any reason not to just check
> WalSndCtl->sync_standbys_defined? rather than both !SyncStandbysDefined()
> and WalSndCtl->sync_standbys_defined?
>
>
> Agree, just checking for WalSndCtl->sync_standbys_defined seems fine.

So the consensus is something like the following? Patch attached.

/*
- * Fast exit if user has not requested sync replication.
+ * Fast exit if user has not requested sync replication, or there are no
+ * sync replication standby names defined.
*/
- if (!SyncRepRequested())
+ if (!SyncRepRequested() ||
+ !((volatile WalSndCtlData *) WalSndCtl)->sync_standbys_defined)
return;

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION