Re: pgsql: Get rid of the dedicated latch for signaling the startup process

On 04/11/2020 15:17, Heikki Linnakangas wrote:
> On 04/11/2020 14:03, Fujii Masao wrote:
>> Or ISTM that WakeupRecovery() should set the latch only when the latch
>> has not been reset to NULL yet.
>
> Got to be careful with race conditions, if the latch is set to NULL at
> the same time that WakeupRecovery() is called.

I don't think commit 113d3591b8 got this quite right:

> void
> WakeupRecovery(void)
> {
> if (XLogCtl->recoveryWakeupLatch)
> SetLatch(XLogCtl->recoveryWakeupLatch);
> }

If XLogCtl->recoveryWakeupLatch is set to NULL between the if and the
SetLatch, you'll still get a segfault. That's highly unlikely to happen
in practice because the compiler will optimize that into a single load
instruction, but could happen with -O0. I think you'd need to do the
access only once, using a volatile pointer, to make that safe. Maybe
it's simpler to just not reset it to NULL, after all.

- Heikki