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

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Message-ID: <8a285fc9-9505-44bf-8286-a83ea78357cc@paulmck-laptop>
Date: Fri, 14 Mar 2025 07:40:50 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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 Fri, Mar 14, 2025 at 01:30:58PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 14, 2025 at 01:00:57PM +0100, Sebastian Andrzej Siewior wrote:
> > 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:

[ . . . ]

> > > > 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.
> 
> Urgh, so we have a rcu_dereference_*() function that does not in fact
> imply rcu_dereference() ? WTF kind of insane naming it that?

My kind of insane naming!  ;-)

The rationale is that "_protected" means "protected from updates".

							Thanx, Paul

------------------------------------------------------------------------

/**
 * rcu_dereference_protected() - fetch RCU pointer when updates prevented
 * @p: The pointer to read, prior to dereferencing
 * @c: The conditions under which the dereference will take place
 *
 * Return the value of the specified RCU-protected pointer, but omit
 * the READ_ONCE().  This is useful in cases where update-side locks
 * prevent the value of the pointer from changing.  Please note that this
 * primitive does *not* prevent the compiler from repeating this reference
 * or combining it with other references, so it should not be used without
 * protection of appropriate locks.
 *
 * This function is only for update-side use.  Using this function
 * when protected only by rcu_read_lock() will result in infrequent
 * but very ugly failures.
 */