linux-kernel - Re: [PATCH locking/Documentation 1/2] Add note of release-acquire store vulnerability

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Message-ID: <20160930085334.GE5016@twins.programming.kicks-ass.net>
Date:   Fri, 30 Sep 2016 10:53:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, mingo@...nel.org, dhowells@...hat.com
Subject: Re: [PATCH locking/Documentation 1/2] Add note of release-acquire
 store vulnerability

On Thu, Sep 29, 2016 at 03:36:38PM -0400, Alan Stern wrote:
> > > If you execute P0 and P1 concurrently and one store of each 'wins' the
> > > LWSYNC of either is null and void, and therefore P2 is unordered and can
> > > observe r2==0.
> 
> Not so.  lwsync instructions cannot be "voided".

I distinctly remember there being a case (smp_wmb()) where the lwsync
would disappear if the store was lost.