Re: Online enabling of checksums

On 2018-04-06 19:59:17 +0200, Tomas Vondra wrote:
> On 04/06/2018 07:46 PM, Andres Freund wrote:
> >> Sure. But what would that be? I can't think of anything. A process that
> >> modifies a buffer (or any other piece of shared state) without holding
> >> some sort of lock seems broken by default.
> > 
> > You can quite possibly already *hold* a lock if it's not an exclusive
> > one.
> > 
> 
> Sure, but if you're holding the buffer lock when the checksum version is
> changed, then the checksumhelper is obviously not running yet. In which
> case it will update the checksum on the buffer later.

The buffer content lock itself doesn't generally give any such guarantee
afaict, as it's required that the content lock is held in shared mode
during IO. ProcessSingleRelationFork() happens to use exclusive mode
(which could and possibly should be optimized), so that's probably
sufficient from that end though.

I'm mainly disconcerted this isn't well discussed & documented.

Greetings,

Andres Freund