Re: GSoC 2017 : Patch for predicate locking in Gist index

Hi, Andrew!

On Mon, Oct 2, 2017 at 1:40 PM, Andrew Borodin <amborodin86@gmail.com>
wrote:

> Thanks for looking into the patch!
>
> On Thu, Sep 28, 2017 at 3:59 PM, Alexander Korotkov <
> a.korotkov@postgrespro.ru> wrote:
>
>>
>>
>> In gistdoinsert() you do CheckForSerializableConflictIn() only if page
>> wasn't exclusively locked before (xlocked is false).
>>
>> if (!xlocked)
>>> {
>>> LockBuffer(stack->buffer, GIST_UNLOCK);
>>> LockBuffer(stack->buffer, GIST_EXCLUSIVE);
>>> CheckForSerializableConflictIn(r, NULL, stack->buffer);
>>> xlocked = true;
>>
>>
>> However, page might be exclusively locked before.  And in this case
>> CheckForSerializableConflictIn() would be skipped.  That happens very
>> rarely (someone fixes incomplete split before we did), but nevertheless.
>>
>
> if xlocked = true, page was already checked for conflict after setting
> exclusive lock on it's buffer.  I still do not see any problem here...
>

What happen if exactly this "continue" fires?

if (GistFollowRight(stack->page))
> {
> if (!xlocked)
> {
> LockBuffer(stack->buffer, GIST_UNLOCK);
> LockBuffer(stack->buffer, GIST_EXCLUSIVE);
> xlocked = true;
> /* someone might've completed the split when we unlocked */
> if (!GistFollowRight(stack->page))
> continue;


In this case we might get xlocked == true without
calling CheckForSerializableConflictIn().  This is very rare codepath, but
still...
I think it would be rather safe and easy for understanding to
more CheckForSerializableConflictIn() directly before gistinserttuple().

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company