linux-kernel - Re: [PATCH v3 2/3] locking: Clarify requirements for smp_mb__after

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Date:   Tue, 3 Jul 2018 19:07:57 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 2/3] locking: Clarify requirements for
 smp_mb__after_spinlock()

On Tue, Jul 03, 2018 at 08:39:10AM -0700, Paul E. McKenney wrote:

[...]

> > + * smp_mb__after_spinlock() provides the equivalent of a full memory barrier
> > + * between program-order earlier lock acquisitions and program-order later
> 
> Not just the earlier lock acquisition, but also all program-order earlier
> memory accesses, correct?

I understand: "but also all program-order earlier memory accesses program-order
before that lock acquisition(s) ...".  Yes, but:

  - I considered this as implied by the above (L ->mb M2 and M1 ->po L implies
    M1 ->mb M2, where M1, M2 are memory accesses and L is a lock acquisition);

  - my prose abilities are limited ;-), and I was/am unable to come up with an
    (to me) acceptable or readable enough way to make it explicit; some ideas?


> > + *	  WRITE_ONCE(X, 1);		WRITE_ONCE(Y, 1);
> > + *	  spin_lock(S);			smp_mb();
> > + *	  smp_mb__after_spinlock();	r1 = READ_ONCE(X);
> > + *	  r0 = READ_ONCE(Y);
> > + *	  spin_unlock(S);
> 
> Should we say that this is an instance of the SB pattern?  (Am OK either
> way, just asking the question.)

I don't think we *should* ;-),  but I'm also OK either way.

  Andrea