linux-kernel - Re: Plain accesses and data races in the Linux Kernel Memory Model

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Message-Id: <20190115152340.GX1215@linux.ibm.com>
Date:   Tue, 15 Jan 2019 07:23:40 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.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.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Plain accesses and data races in the Linux Kernel Memory Model

On Tue, Jan 15, 2019 at 10:03:26AM -0500, Alan Stern wrote:
> On Tue, 15 Jan 2019, Dmitry Vyukov wrote:
> 
> > On Tue, Jan 15, 2019 at 12:54 AM Paul E. McKenney <paulmck@...ux.ibm.com> wrote:
> > >
> > > On Mon, Jan 14, 2019 at 02:41:49PM -0500, Alan Stern wrote:
> > > > The patch below is my first attempt at adapting the Linux Kernel
> > > > Memory Model to handle plain accesses (i.e., those which aren't
> > > > specially marked as READ_ONCE, WRITE_ONCE, acquire, release,
> > > > read-modify-write, or lock accesses).  This work is based on an
> > > > initial proposal created by Andrea Parri back in December 2017,
> > > > although it has grown a lot since then.
> > >
> > > Hello, Alan,
> > >
> > > Good stuff!!!
> > >
> > > I tried applying this in order to test it against the various litmus
> > > tests, but no joy.  Could you please tell me what commit is this patch
> > > based on?
> > >
> > >                                                         Thanx, Paul
> > >
> > > > The adaptation involves two main aspects: recognizing the ordering
> > > > induced by plain accesses and detecting data races.  They are handled
> > > > separately.  In fact, the code for figuring out the ordering assumes
> > > > there are no data races (the idea being that if a data race is
> > > > present then pretty much anything could happen, so there's no point
> > > > worrying about it -- obviously this will have to be changed if we want
> > > > to cover seqlocks).
> > 
> > Hi Alan,
> > 
> > Is there a mailing list dedicated to this effort? Private messages
> > tend to lost over time, no archive, not possible to send a link or
> > show full history to anybody, etc.
> 
> No specific mailing list.  We've been relying on LKML.
> 
> > Re seqlocks, strictly saying defining races for seqlocks is not
> > necessary. Seqlocks can be expressed without races in C by using
> > relaxed atomic loads within the read critical section. We may consider
> > this option as well.
> 
> That seems like a reasonable approach.

What Alan said!  ;-)

							Thanx, Paul