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

[Peter Zijlstra]

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.