linux-kernel - Re: [PATCH 3/5] asm-generic, kcsan: Add KCSAN instrumentation for bitops

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Message-ID: <20200121161632.GV2935@paulmck-ThinkPad-P72>
Date:   Tue, 21 Jan 2020 08:16:32 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, andreyknvl@...gle.com,
        glider@...gle.com, dvyukov@...gle.com, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, mark.rutland@....com,
        will@...nel.org, boqun.feng@...il.com, arnd@...db.de,
        viro@...iv.linux.org.uk, christophe.leroy@....fr, dja@...ens.net,
        mpe@...erman.id.au, rostedt@...dmis.org, mhiramat@...nel.org,
        mingo@...nel.org, christian.brauner@...ntu.com,
        daniel@...earbox.net, cyphar@...har.com, keescook@...omium.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH 3/5] asm-generic, kcsan: Add KCSAN instrumentation for
 bitops

On Tue, Jan 21, 2020 at 03:47:16PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2020 at 06:21:09AM -0800, Paul E. McKenney wrote:
> > On Tue, Jan 21, 2020 at 10:15:01AM +0100, Peter Zijlstra wrote:
> > > On Mon, Jan 20, 2020 at 12:23:59PM -0800, Paul E. McKenney wrote:
> > > > We also don't have __atomic_read() and __atomic_set(), yet atomic_read()
> > > > and atomic_set() are considered to be non-racy, right?
> > > 
> > > What is racy? :-) You can make data races with atomic_{read,set}() just
> > > fine.
> > 
> > Like "fairness", lots of definitions of "racy".  ;-)
> > 
> > > Anyway, traditionally we call the read-modify-write stuff atomic, not
> > > the trivial load-store stuff. The only reason we care about the
> > > load-store stuff in the first place is because C compilers are shit.
> > > 
> > > atomic_read() / test_bit() are just a load, all we need is the C
> > > compiler not to be an ass and split it. Yes, we've invented the term
> > > single-copy atomicity for that, but that doesn't make it more or less of
> > > a load.
> > > 
> > > And exactly because it is just a load, there is no __test_bit(), which
> > > would be the exact same load.
> > 
> > Very good!  Shouldn't KCSAN then define test_bit() as non-racy just as
> > for atomic_read()?
> 
> Sure it does; but my comment was aimed at the gripe that test_bit()
> lives in the non-atomic bitops header. That is arguably entirely
> correct.

Fair enough!

							Thanx, Paul