Re: Online enabling of checksums

On 04/06/2018 08:13 PM, Andres Freund wrote:
> 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.
> 

Yes.

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

Agreed, no argument here.


-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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