linux-kernel - Re: [PATCH v10 00/21] futex: Add support task local hash maps, FUTEX2_NUMA and FUTEX2

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250314104259.GA36322@noisy.programming.kicks-ass.net>
Date: Fri, 14 Mar 2025 11:42:59 +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 v10 00/21] futex: Add support task local hash maps,
 FUTEX2_NUMA and FUTEX2_MPOL

On Wed, Mar 12, 2025 at 04:18:48PM +0100, Sebastian Andrzej Siewior wrote:
> @@ -196,12 +196,12 @@ static bool __futex_pivot_hash(struct mm_struct *mm,
>  {
>  	struct futex_private_hash *fph;
>  
> -	lockdep_assert_held(&mm->futex_hash_lock);
>  	WARN_ON_ONCE(mm->futex_phash_new);
>  
> -	fph = mm->futex_phash;
> +	fph = rcu_dereference_protected(mm->futex_phash,
> +					lockdep_is_held(&mm->futex_hash_lock));

I are confused... this makes no sense. Why ?!

We only ever write that variable while holding this lock, we hold the
lock, we don't need RCU to read the variable.