linux-kernel - Re: f68f031d ("Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}

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Message-ID: <20190503163411.GH2606@hirez.programming.kicks-ass.net>
Date:   Fri, 3 May 2019 18:34:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: f68f031d ("Documentation: atomic_t.txt: Explain ordering
 provided by smp_mb__{before,after}_atomic()")

On Fri, May 03, 2019 at 12:19:21PM -0400, Alan Stern wrote:
> On Fri, 3 May 2019, Peter Zijlstra wrote:
> 
> > On Fri, May 03, 2019 at 07:53:26AM -0700, Paul E. McKenney wrote:
> > > Hello, Alan,
> > > 
> > > Just following up on the -rcu commit below.  I believe that it needs
> > > some adjustment given Peter Zijlstra's addition of "memory" to the x86
> > > non-value-returning atomics, but thought I should double-check.
> > 
> > Right; I should get back to that thread...
> 
> The real question, still outstanding, is whether smp_mb__before_atomic 
> orders anything following the RMW instruction (and similarly, whether 
> smp_mb__after_atomic orders anything preceding the RMW instruction).

Yes -- that was very much the intent, and only (some) x86 ops and (some)
MIPS config have issues with that.