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

[Paul E. McKenney]

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()?

							Thanx, Paul