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

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Date:   Mon, 2 Jul 2018 14:50:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.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>,
        "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, Jun 28, 2018 at 07:30:45PM +0200, Andrea Parri wrote:
> On Thu, Jun 28, 2018 at 05:05:50PM +0200, Peter Zijlstra wrote:
> > On Thu, Jun 28, 2018 at 12:41:19PM +0200, Andrea Parri wrote:
> > > - * 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.
> > 
> > Please don't remove that; that explains _why_ we need a full memory
> > barrier here.
> 
> Peter:
> 
> Both you and Boqun stated that the above snippet is "bad":
> 
>   http://lkml.kernel.org/r/20180312085646.GE4064@hirez.programming.kicks-ass.net
> 
> and I do agree with your assessment! ;-)

Right..

> I've no objection to keep that comment (together with the
> "clarification" suggested in this patch) _once_ replaced
> that snippet with something else (say, with the snippet
> Boqun suggested in:
> 
>   http://lkml.kernel.org/r/20180312085600.aczjkpn73axzs2sb@tardis ):
> 
> is this what you mean?

Yes. I much prefer to explain the why for rule than to just state them.