Re: [PATCH 1/3] selftests/futex: Adapt the private hash test to RCU related changes

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 07, 2025 at 04:36:21PM +0200, Sebastian Andrzej Siewior wrote:
> The auto scaling on create creation used to automatically assign the new
> hash because there was the private hash was unused and could be replaced
> right away.
> 
> With the upcoming change to wait for a RCU grace period before the hash
> can be assigned, the test will always fail.
> 
> If the reported number of hash buckets is not updated after an
> auto scaling event, block on an acquired lock with a timeout. The timeout
> is the delay to wait towards a grace period and locking and a locked
> pthread_mutex_t ensure that glibc calls into kernel using futex
> operation which will assign new private hash if available.
> This will retry every 100ms up to 2 seconds in total.

So the auto scaling thing is 'broken' in that if you do a 'final'
pthread_create() it will try and stage this new hash. If for whatever
reason the refcount isn't '0' -- and this can already happen today due
to a concurrent futex operation. Nothing will re-try the rehash.

This RCU business made this all but a certainty, but the race was there.

I briefly wondered if we should have a workqueue re-try the rehash.