Re: Connections hang indefinitely while taking a gin index's LWLock buffer_content lock

Hi!

Mail.App somehow borken my reponse with <div class="ApplePlainTextBody">, which is far beyond of my understanding of what is plain text, that's why I'll quote all my previous message here. Hope this mail client is OK.

14.11.2018, 23:13, "Andrey Borodin" <x4mmm(at)yandex-team(dot)ru>:
> Hi everyone!
>
> I didn't noticed this thread for too long somehow, sorry.
>
>> 8 нояб. 2018 г., в 6:46, Peter Geoghegan <pg(at)bowt(dot)ie> написал(а):
>>
>> I don't think
>> the general "there can't be any inserters at this subtree" thing works
>> given that we have to couple buffer locks when moving right for other
>> reasons. We call ginStepRight() within ginFinishSplit(), for reasons
>> that date back to bug fix commit ac4ab97e from 2013 -- that detail is
>> probably important, because it seems to be what breaks the subtree
>> design (we don't delete in two phases or anything in GIN).
>
> ginVacuumPostingTreeLeaves() holds LockBufferForCleanup() on subtree root b.
> Thus there may be no GinBtreeStack's holding pin on b at the moment.
> When you ginStepRight(b) to the parent in ginFinishSplit(), you always get to the buffer from your stack.
> Hence you can never have ginFinishSplit() deadlock with cleanup of subtree whose root is LockBufferForCleanup()'d.
>
> Is this correct or did I miss something?
>
> But we have a deadlock at hand, I'll think more about it. Something with locking protocol is clearly wrong.
>
>> 11 нояб. 2018 г., в 22:33, chenhj <chjischj(at)163(dot)com> написал(а):
>>
>> The order of get lwlock in ginRedoDeletePage() may should be change from "dbuffer->pbuffer->lbuffer" to "lbuffer->dbuffer->pbuffer" . Is this right?
>
> This looks correct to me.
>
> Best regards, Andrey Borodin.

I've reread Chen's reports and understood his findings.

> When you ginStepRight(b) to the parent in ginFinishSplit(), you always get to the buffer from your stack.

This statement does not hold.
When you have a GinBtreeStack S, one of it's internal pages may split into new page P. If in this moment you start a multi-level delete from P and cascade split from S, P is not in S and we may deadlock.

Correct solution seems to replace lock-coupling descent by usual B-tree searches for parent pages as in B-tree.
I think I can compose patch for consideration.

Best regards, Andrey Borodin.