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

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Message-ID: <Pine.LNX.4.44L0.1909131509250.1466-100000@iolanthe.rowland.org>
Date:   Fri, 13 Sep 2019 15:13:26 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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).


Alan