linux-kernel - Re: [PATCH] tools/memory-model: document the "one-time init" pattern

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Date:   Mon, 20 Jul 2020 15:06:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     peterz@...radead.org
Cc:     Matthew Wilcox <willy@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Dave Chinner <david@...morbit.com>,
        Eric Biggers <ebiggers@...nel.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern

On Mon, Jul 20, 2020 at 06:48:50PM +0200, peterz@...radead.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