linux-kernel - Re: [PATCH v12 00/21] futex: Add support task local hash maps, FUTEX2_NUMA and FUTEX2

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Message-ID: <20250509114105.HZdDPPO5@linutronix.de>
Date: Fri, 9 May 2025 13:41:05 +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 v12 00/21] futex: Add support task local hash maps,
 FUTEX2_NUMA and FUTEX2_MPOL

On 2025-05-06 09:36:11 [+0200], Peter Zijlstra wrote:
> Well, if you do stupid things, you get to keep the pieces or something
> along those lines. Same as when userspace goes scribble the node value
> while another thread is waiting and all that.
> 
> Even with the unconditional write back you're going to have a problem
> with concurrent wait on the same futex.

We could add a global lock for the write back case to ensure there is
only one at a time. However let me document the current behaviour of the
new pieces and tick it off ;)

Sebastian