linux-kernel - Re: [PATCH v8 00/15] futex: Add support task local hash maps.

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Message-ID: <20250220151206.GB11590@noisy.programming.kicks-ass.net>
Date: Thu, 20 Feb 2025 16:12:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 v8 00/15] futex: Add support task local hash maps.

On Wed, Feb 05, 2025 at 01:20:26PM +0100, Sebastian Andrzej Siewior wrote:

> So the beauty of it is that you enforce a ref drop on hb once it gets
> out of scope. So you can't use it by chance once the ref is dropped.
> 
> But this does not help in futex_lock_pi() where you have the drop the
> reference before __rt_mutex_start_proxy_lock() (or at least before
> rt_mutex_wait_proxy_lock()) but still have it you go for the no_block
> shortcut. At which point even the lock is still owned.
> 
> While it makes the other cases nicer, the futex_lock_pi() function was
> the only one where I was thinking about setting hb to NULL to avoid
> accidental usage later on.

Sorry for the delay.. got distracted :/

I think we can simply put:

	futex_hash_put(no_free_ptr(hb));

right where we drop hb->lock in futex_lock_pi().

I've split up the patch a little and stuck them here:

  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=futex/scope