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

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Message-ID: <20250314120057.NcjcFp3K@linutronix.de>
Date: Fri, 14 Mar 2025 13:00:57 +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 12:41:02 [+0100], Peter Zijlstra wrote:
> On Fri, Mar 14, 2025 at 12:28:08PM +0100, Sebastian Andrzej Siewior wrote:
> > 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.
> 
> Yeah, but sparse is stupid.

I though we like sparse.

> > This makes it obvious that you can access it, it won't change as long
> > as you have the lock.
> 
> It's just plain confusing. rcu_dereference() says you care about the
> load being single copy atomic and the data dependency, we don't.
> 
> If we just want to shut up sparse; can't we write it like:
> 
> 	cur = unrcu_pointer(mm->futex_phash);
> 
> ?

But isn't rcu_dereference_protected() doing exactly this? It only
verifies that lockdep_is_held() thingy and it performs a plain read, no
READ_ONCE() or anything. And the reader understands why it is safe to
access the pointer as-is.

Sebastian