Re: Issue with the PRNG used by Postgres

I wrote:
> I don't think it's correct to re-initialize the SpinDelayStatus each
> time around the outer loop. That state should persist through the
> entire acquire operation, as it does in a regular spinlock acquire.
> As this stands, it resets the delay to minimum each time around the
> outer loop, and I bet it is that behavior not the RNG that's to blame
> for what he's seeing.

After thinking about this some more, it is fairly clear that that *is*
a mistake that can cause a thundering-herd problem. Assume we have
two or more backends waiting in perform_spin_delay, and for whatever
reason the scheduler wakes them up simultaneously. They see the
LW_FLAG_LOCKED bit clear (because whoever had the lock when they went
to sleep is long gone) and iterate the outer loop. One gets the lock;
the rest go back to sleep. And they will all sleep exactly
MIN_DELAY_USEC, because they all reset their SpinDelayStatus.
Lather, rinse, repeat. If the s_lock code were being used as
intended, they would soon acquire different cur_delay settings;
but that never happens. That is not the RNG's fault.

So I think we need something like the attached.

regards, tom lane