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

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Message-ID: <Pine.LNX.4.44L0.1909161122390.1489-100000@iolanthe.rowland.org>
Date:   Mon, 16 Sep 2019 11:25:07 -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:

> > In other words, we can define ->vis as:
> > 
> > 	let vis = prop ; ((strong-fence ; [Marked] ; xbstar) | (xbstar & int))
> > 
> 
> Hmm.. so the problem with this approach is that the (xbstar & int) part
> doesn't satisfy the requirement of visibility... i.e.
> 
> 	X ->prop Z ->(xbstar & int) Y
> 
> may not guarantee when Y executes, X is already propagated to Y's CPU.

Yes, it doesn't guarantee this.  But the reason it doesn't guarantee 
this is because of the prop.  The (xbstar & int) part is okay.  In 
other words, if

	Z ->(xbstar & int) Y

then it is certainly true that any store propagating to Z's CPU before 
Z executes also propagates to Y's CPU (which is the same one) before Y 
executes.

Alan