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

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Message-ID: <20250708080140.YUpDy-bM@linutronix.de>
Date: Tue, 8 Jul 2025 10:01:40 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 1/3] selftests/futex: Adapt the private hash test to RCU
 related changes

On 2025-07-08 09:41:24 [+0200], Peter Zijlstra wrote:
> 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.

Sure it was there but not in the way the test case was setup. I *think*
it is okay because in real life the new hash will be put to use unless
you terminate shortly after at which point you don't need it.

> 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.

I don't think we need to worry about it.

Sebastian