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

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Message-ID: <20250314112808.-XVssA31@linutronix.de>
Date: Fri, 14 Mar 2025 12:28:08 +0100
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 v10 00/21] futex: Add support task local hash maps,
 FUTEX2_NUMA and FUTEX2_MPOL

On 2025-03-14 11:58:56 [+0100], Peter Zijlstra wrote:
> On Wed, Mar 12, 2025 at 04:18:48PM +0100, Sebastian Andrzej Siewior wrote:
> 
> > @@ -1591,7 +1597,8 @@ static int futex_hash_allocate(unsigned int hash_slots, bool custom)
> >  		struct futex_private_hash *free __free(kvfree) = NULL;
> >  		struct futex_private_hash *cur, *new;
> >  
> > -		cur = mm->futex_phash;
> > +		cur = rcu_dereference_protected(mm->futex_phash,
> > +						lockdep_is_held(&mm->futex_hash_lock));
> >  		new = mm->futex_phash_new;
> >  		mm->futex_phash_new = NULL;
> >  
> 
> Same thing again, this makes no sense.

With "mm->futex_phash" sparse complains about direct RCU access. This
makes it obvious that you can access it, it won't change as long as you
have the lock.

Sebastian