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

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Message-ID: <20200120165223.GC14914@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Jan 2020 17:52:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.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 Mon, Jan 20, 2020 at 08:27:25AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 20, 2020 at 03:40:48PM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 20, 2020 at 03:19:25PM +0100, Marco Elver wrote:
> > > Add explicit KCSAN checks for bitops.
> > > 
> > > Note that test_bit() is an atomic bitop, and we instrument it as such,
> > 
> > Well, it is 'atomic' in the same way that atomic_read() is. Both are
> > very much not atomic ops, but are part of an interface that facilitates
> > atomic operations.
> 
> True, but they all are either inline assembly or have either an
> implicit or explicit cast to volatile, so they could be treated
> the same as atomic_read(), correct?  If not, what am I missing?

Sure, but that is due to instrumentation requirements, not anything
else.

Also note the distinct lack of __test_bit(), to mirror the non-atomic
__set_bit() and __clear_bit().