Re: LogwrtResult contended spinlock

On Sat, Apr 6, 2024 at 9:21 AM Thomas Munro <thomas.munro@gmail.com> wrote:
>
> On Sat, Apr 6, 2024 at 6:55 AM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> > Pushed 0001.
>
> Could that be related to the 3 failures on parula that look like this?
>
> TRAP: failed Assert("node->next == 0 && node->prev == 0"), File:
> "../../../../src/include/storage/proclist.h", Line: 63, PID: 29119
> 2024-04-05 16:16:26.812 UTC [29114:15] pg_regress/drop_operator LOG:
> statement: DROP OPERATOR <|(bigint, bigint);
> postgres: postgres regression [local] CREATE
> ROLE(ExceptionalCondition+0x4c)[0x9b3fdc]
> postgres: postgres regression [local] CREATE ROLE[0x8529e4]
> postgres: postgres regression [local] CREATE
> ROLE(LWLockWaitForVar+0xec)[0x8538fc]
> postgres: postgres regression [local] CREATE ROLE[0x54c7d4]
> postgres: postgres regression [local] CREATE ROLE(XLogFlush+0xf0)[0x552600]
> postgres: postgres regression [local] CREATE ROLE[0x54a9b0]
> postgres: postgres regression [local] CREATE ROLE[0x54bbdc]
>
> Hmm, the comments for LWLockWaitForVar say:
>
>  * Be aware that LWLockConflictsWithVar() does not include a memory barrier,
>  * hence the caller of this function may want to rely on an explicit barrier or
>  * an implied barrier via spinlock or LWLock to avoid memory ordering issues.
>
> But that seems to be more likely to make LWLockWaitForVar suffer data
> races (ie hang), not break assertions about LWLock sanity, so I don't
> know what's going on there.  I happened to have a shell on a Graviton
> box, but I couldn't reproduce it after a while...

Thanks for reporting. I'll try to spin up a similar instance like
parula and reproduce. Meanwhile, I'm wondering if it is somehow
related to what's discussed in "Why is parula failing?"
https://www.postgresql.org/message-id/4009739.1710878318%40sss.pgh.pa.us.
It seems like parula is behaving unexpectedly because of the compiler
and other stuff.

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com