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

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Message-ID: <Pine.LNX.4.44L0.1806280947080.1812-100000@iolanthe.rowland.org>
Date:   Thu, 28 Jun 2018 09:49:58 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrea Parri <andrea.parri@...rulasolutions.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>,
        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>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 2/3] locking: Clarify requirements for smp_mb__after_spinlock()

On Thu, 28 Jun 2018, Andrea Parri wrote:

> --- a/include/linux/spinlock.h
> +++ b/include/linux/spinlock.h
> @@ -114,29 +114,8 @@ do {								\
>  #endif /*arch_spin_is_contended*/
>  
>  /*
> - * This barrier must provide two things:
> - *
> - *   - it must guarantee a STORE before the spin_lock() is ordered against a
> - *     LOAD after it, see the comments at its two usage sites.
> - *
> - *   - it must ensure the critical section is RCsc.
> - *
> - * The latter is important for cases where we observe values written by other
> - * CPUs in spin-loops, without barriers, while being subject to scheduling.
> - *
> - * CPU0			CPU1			CPU2
> - *
> - *			for (;;) {
> - *			  if (READ_ONCE(X))
> - *			    break;
> - *			}
> - * X=1
> - *			<sched-out>
> - *						<sched-in>
> - *						r = X;
> - *
> - * without transitivity it could be that CPU1 observes X!=0 breaks the loop,
> - * we get migrated and CPU2 sees X==0.
> + * smp_mb__after_spinlock() provides a full memory barrier between po-earlier
> + * lock acquisitions and po-later memory accesses.

How about saying "provides the equivalent of a full memory barrier"?

The point being that smp_mb__after_spinlock doesn't have to provide an
actual barrier; it just has to ensure the behavior is the same as if a
full barrier were present.

Alan