Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Mon, 9 Mar 2020 at 15:50, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Mar 9, 2020 at 11:38 AM Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>>
>> On Mon, 9 Mar 2020 at 14:16, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> >
>> > On Sun, Mar 8, 2020 at 7:58 AM Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>> >> >
>> >> > Fair position, as per initial analysis, I think if we do below three
>> >> > things, it should work out without changing to a new way of locking
>> >> > for relation extension or page type locks.
>> >> > a. As per the discussion above, ensure in code we will never try to
>> >> > acquire another heavy-weight lock after acquiring relation extension
>> >> > or page type locks (probably by having Asserts in code or maybe some
>> >> > other way).
>> >>
>> >> The current patch
>> >> (v02_0001-Added-assert-to-verify-that-we-never-try-to-take-any.patch)
>> >> doesn't check that acquiring a heavy-weight lock after page type lock,
>> >> is that right?
>> >
>> >
>> > No, it should do that.
>> >
>> >>
>> >> There is the path doing that: ginInsertCleanup() holds
>> >> a page lock and insert the pending list items, which might hold a
>> >> relation extension lock.
>> >
>> >
>> > Right, I could also see that, but do you see any problem with that? I agree that Assert should cover this case, but I don't see any fundamental problem with that.
>>
>> I think that could be a problem if we change the group locking so that
>> it doesn't consider page lock type.
>
>
> I might be missing something, but won't that be a problem only when if there is a case where we acquire page lock after acquiring a relation extension lock?

Yes, you're right.

Well I meant that the reason why we need to make Assert should cover
page locks case is the same as the reason for extension lock type
case. If we change the group locking so that it doesn't consider
extension lock and change deadlock so that it doesn't make a wait edge
for it, we need to ensure that the same backend doesn't acquire
heavy-weight lock after holding relation extension lock. These are
already done in the current patch. Similarly, if we did the similar
change for page lock in the group locking and deadlock , we need to
ensure the same things for page lock. But ISTM it doesn't necessarily
need to support page lock for now because currently we use it only for
cleanup pending list of gin index.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
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