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

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Message-ID: <Pine.LNX.4.44L0.1909161119160.1489-100000@iolanthe.rowland.org>
Date:   Mon, 16 Sep 2019 11:22:18 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Boqun Feng <boqun.feng@...il.com>
cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <parri.andrea@...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>,
        "Paul E. McKenney" <paulmck@...ux.ibm.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 Mon, 16 Sep 2019, Boqun Feng wrote:

> > executes if Y is a store.)  This is expressed by the visibility
> > relation (vis), where X ->vis Y is defined to hold if there is an
> > intermediate event Z such that:
> > 
> > 	X is connected to Z by a possibly empty sequence of
> > 	cumul-fence links followed by an optional rfe link (if none of
> > 	these links are present, X and Z are the same event),
> > 
> 
> I wonder whehter we could add an optional ->coe or ->fre between X and
> the possibly empty sequence of cumul-fence, smiliar to the definition
> of ->prop. This makes sense because if we have
> 
> 	X ->coe X' (and X' in on CPU C)
> 
> , X must be already propagated to C before X' executed, according to our
> operation model:
> 
> "... In particular, it must arrange for the store to be co-later than
> (i.e., to overwrite) any other store to the same location which has
> already propagated to CPU C."

You have misinterpreted the text.  The operational model says that if X 
propagates to CPU C before X' executes then X ->coe X'.  It does _not_ 
say that if X ->coe X' then X propagates to CPU C before X' executes.

> In other words, we can define ->vis as:
> 
> 	let vis = prop ; ((strong-fence ; [Marked] ; xbstar) | (xbstar & int))
> 
> , and for this document, reference the "prop" section in
> explanation.txt. This could make the model simple (both for description
> and explanation), and one better thing is that we won't need commit in
> Paul's dev branch:
> 	
> 	c683f2c807d2 "tools/memory-model: Fix data race detection for unordered store and load"
> 
> , and data race rules will look more symmetrical ;-)
> 
> Thoughts? Or am I missing something subtle here?

See above.

Alan