linux-kernel - RE: [PATCH v2] tools/memory-model/Documentation: Fix "conflict" definition

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Date:   Mon, 2 Mar 2020 17:44:11 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Marco Elver' <elver@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "will@...nel.org" <will@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: RE: [PATCH v2] tools/memory-model/Documentation: Fix "conflict"
 definition

From: Marco Elver
> Sent: 02 March 2020 14:18
> 
> The definition of "conflict" should not include the type of access nor
> whether the accesses are concurrent or not, which this patch addresses.
> The definition of "data race" remains unchanged.
> 
> The definition of "conflict" as we know it and is cited by various
> papers on memory consistency models appeared in [1]: "Two accesses to
> the same variable conflict if at least one is a write; two operations
> conflict if they execute conflicting accesses."

I'm pretty sure that Linux requires that the underlying memory
subsystem remove any possible 'conflicts' by serialising the
requests (in an arbitrary order).

So 'conflicts' are never relevant.

There are memory subsystems where conflicts MUST be avoided.
For instance the fpga I use have some dual-ported memory.
Concurrent accesses on the two ports for the same address
must (usually) be avoided if one is a write.
Two writes will generate corrupt memory.
A concurrent write+read will generate a garbage read.
In the special case where the two ports use the same clock
it is possible to force the read to be 'old data' but that
constrains the timings.

On such systems the code must avoid conflicting cycles.

	David

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