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From: "Paul E. McKenney" <paulmck@kernel.org>
To: peterz@infradead.org
Cc: Matthew Wilcox <willy@infradead.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Dave Chinner <david@fromorbit.com>,
	Eric Biggers <ebiggers@kernel.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, Akira Yokosawa <akiyks@gmail.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	"Darrick J . Wong" <darrick.wong@oracle.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Nicholas Piggin <npiggin@gmail.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern
Date: Mon, 20 Jul 2020 15:06:51 -0700
Message-ID: <20200720220651.GV9247@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200720164850.GF119549@hirez.programming.kicks-ass.net>

On Mon, Jul 20, 2020 at 06:48:50PM +0200, peterz@infradead.org wrote:
> On Mon, Jul 20, 2020 at 09:04:34AM -0700, Paul E. McKenney wrote:
> > 2.	If we were to say "unlock" instead of "release", consistency
> > 	would demand that we also say "lock" instead of "acquire".
> > 	But "lock" is subtlely different than "acquire", and there is
> > 	a history of people requesting further divergence.
> 
> This, acquire/release are RCpc, while (with the exception of Power)
> LOCK/UNLOCK are RCsc.
> 
> ( Or did we settle on RCtso for our release/acquire order? I have vague
> memories of a long-ish thread, but seem to have forgotten the outcome,
> if any. )
> 
> Lots of subtlety and head-aches right about there. Anyway, it would be
> awesome if we can get Power into the RCsc locking camp :-)

I will let you take that one up with the Power folks.

But I should give an example of a current difference between lock and
acquire, and just to spread the blame, I will pick on an architecture
other than Power.  ;-)

With lock acquisition, the following orders the access to X and Z:

	WRITE_ONCE(X, 1);
	spin_lock(&my_lock);
	smp_mb__after_lock();
	r1 = READ_ONCE(Z);

But if we replace the lock acquisition with a load acquire, there are
no ordering guarantees for the accesses to X and Z:

	WRITE_ONCE(X, 1);
	r2 = smp_load_acquire(&Y);
	smp_mb__after_lock();  // Yeah, there is no lock.  ;-)
	r3 = READ_ONCE(Z);

There -is- ordering between the accesses to Y and Z, but not to X and Z.
This is not a theoretical issue.  The x86 platform really can reorder
the access to X to follow that of both Y and Z.

So the memory-model divergence between lock acquisition and acquire
loads is very real in the here and now.

							Thanx, Paul

  reply index

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17  4:44 Eric Biggers
2020-07-17  5:49 ` Sedat Dilek
2020-07-17 12:35 ` Matthew Wilcox
2020-07-17 14:26 ` Alan Stern
2020-07-17 17:47 ` Matthew Wilcox
2020-07-17 17:51   ` Alan Stern
2020-07-18  1:02     ` Eric Biggers
2020-07-27 12:51       ` Matthew Wilcox
2020-07-17 21:05   ` Darrick J. Wong
2020-07-18  0:44   ` Darrick J. Wong
2020-07-18  1:38   ` Eric Biggers
2020-07-18  2:13     ` Matthew Wilcox
2020-07-18  5:28       ` Eric Biggers
2020-07-18 14:35         ` Alan Stern
2020-07-20  2:07         ` Dave Chinner
2020-07-20  9:00           ` Peter Zijlstra
2020-07-27 15:17         ` Alan Stern
2020-07-27 15:28           ` Matthew Wilcox
2020-07-27 16:01             ` Paul E. McKenney
2020-07-27 16:31             ` Alan Stern
2020-07-27 16:59               ` Matthew Wilcox
2020-07-27 19:13                 ` Alan Stern
2020-07-17 20:53 ` Darrick J. Wong
2020-07-18  0:58   ` Eric Biggers
2020-07-18  1:25     ` Alan Stern
2020-07-18  1:40       ` Matthew Wilcox
2020-07-18  2:00       ` Dave Chinner
2020-07-18 14:21         ` Alan Stern
2020-07-18  2:00       ` Eric Biggers
2020-07-18  1:42 ` Dave Chinner
2020-07-18 14:08   ` Alan Stern
2020-07-20  1:33     ` Dave Chinner
2020-07-20 14:52       ` Alan Stern
2020-07-20 15:37         ` Darrick J. Wong
2020-07-20 15:39         ` Matthew Wilcox
2020-07-20 16:04           ` Paul E. McKenney
2020-07-20 16:48             ` peterz
2020-07-20 22:06               ` Paul E. McKenney [this message]
2020-07-20 16:12           ` Alan Stern

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