Thread

Re: injection_points: Switch wait/wakeup to use atomics rather than latches

Andrey Borodin <x4mmm@yandex-team.ru> — 2026-05-30T08:05:25Z

> On 28 May 2026, at 07:43, Michael Paquier <michael@paquier.xyz> wrote:
> 
> Andrey in CC, as I'm sure he is interested in that.

Thanks! That's exactly what I need for my tests.

> On 29 May 2026, at 18:31, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> 
>> I'm still struggling to understand. Condition variables and latches
>> are both designed to allow for nice waits and wakeups.
> 
> They only work after you have a PGPROC slot. If you want to inject code to authentication, or into postmaster, you cannot use them.

I have another reason: postmaster death behavior. When we wait on
ConVar and postmaster is kill-9-ed, we release all LWLocks. Which causes
corruption [0], because checkpointer can flush something that's not in WAL.

So I'm trying to build corruption-seeking tests using tool that can induce corruption
in tests.

About the patch:
- inj_state->wait_counts[index]++;
SpinLockRelease(&inj_state->lock);

- /* And broadcast the change to the waiters */
- ConditionVariableBroadcast(&inj_state->wait_point);
+ pg_atomic_fetch_add_u32(&inj_state->wait_counts[index], 1);

Can we move pg_atomic_fetch_add_u32() back under the lock?
We determine slot index under lock, then wakeup slot outside the lock.
In a correctly written test meaning this is not a problem.
However, technically, identity of a slot can change between releasing the lock
and incrementing wait_counts[index].

I'll do another pass tomorrow, maybe something else will catch my eye.


Best regards, Andrey Borodin.

[0] https://www.postgresql.org/message-id/B3C69B86-7F82-4111-B97F-0005497BB745%40yandex-team.ru