linux-kernel - Re: Documentation for plain accesses and data races

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Date:   Sun, 15 Sep 2019 16:13:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Documentation for plain accesses and data races

On Fri, Sep 13, 2019 at 03:13:26PM -0400, Alan Stern wrote:
> On Thu, 12 Sep 2019, Paul E. McKenney wrote:
> 
> > On Fri, Sep 06, 2019 at 02:11:29PM -0400, Alan Stern wrote:
> 
> > > To this end, the LKMM imposes three extra restrictions, together
> > > called the "plain-coherence" axiom because of their resemblance to the
> > > coherency rules:
> > > 
> > > 	If R and W conflict and it is possible to link R to W by one
> > > 	of the xb* sequences listed above, then W ->rfe R is not
> > > 	allowed (i.e., a load cannot read from a store that it
> > > 	executes before, even if one or both is plain).
> > > 
> > > 	If W and R conflict and it is possible to link W to R by one
> > > 	of the vis sequences listed above, then R ->fre W is not
> > > 	allowed (i.e., if a store is visible to a load then the load
> > > 	must read from that store or one coherence-after it).
> > > 
> > > 	If W and W' conflict and it is possible to link W to W' by one
> > > 	of the vis sequences listed above, then W' ->co W is not
> > > 	allowed (i.e., if one store is visible to another then it must
> > > 	come after in the coherence order).
> 
> > I will need to read this last section again.  Perhaps more than once.  ;-)
> 
> I decided this part could use some improvement.  Here is the updated
> text:
> 
> 
> To this end, the LKMM imposes three extra restrictions, together
> called the "plain-coherence" axiom because of their resemblance to the
> rules used by the operational model to ensure cache coherence (that
> is, the rules governing the memory subsystem's choice of a store to
> satisfy a load request and its determination of where a store will
> fall in the coherence order):
> 
> 	If R and W conflict and it is possible to link R to W by one
> 	of the xb* sequences listed above, then W ->rfe R is not
> 	allowed (i.e., a load cannot read from a store that it
> 	executes before, even if one or both is plain).
> 
> 	If W and R conflict and it is possible to link W to R by one
> 	of the vis sequences listed above, then R ->fre W is not
> 	allowed (i.e., if a store is visible to a load then the load
> 	must read from that store or one coherence-after it).
> 
> 	If W and W' conflict and it is possible to link W to W' by one
> 	of the vis sequences listed above, then W' ->co W is not
> 	allowed (i.e., if one store is visible to a second then the
> 	second must come after the first in the coherence order).

These look good to me!

							Thanx, Paul