Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

[Marco Elver]

On Tue, 12 May 2020 at 23:15, Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Tue, May 12, 2020 at 09:07:55PM +0200, Peter Zijlstra wrote:
> > On Tue, May 12, 2020 at 07:53:00PM +0200, Marco Elver wrote:
> > > I just ran a bunch of KCSAN tests. While this series alone would have
> > > passed the tests, there appears to be a problem with
> > > __READ_ONCE/__WRITE_ONCE. I think they should already be using
> > > 'data_race()', as otherwise we will get lots of false positives in
> > > future.
> > >
> > > I noticed this when testing -tip/locking/kcsan, which breaks
> > > unfortunately, because I see a bunch of spurious data races with
> > > arch_atomic_{read,set} because "locking/atomics: Flip fallbacks and
> > > instrumentation" changed them to use __READ_ONCE()/__WRITE_ONCE().
> > > From what I see, the intent was to not double-instrument,
> > > unfortunately they are still double-instrumented because
> > > __READ_ONCE/__WRITE_ONCE doesn't hide the access from KCSAN (nor KASAN
> > > actually). I don't think we can use __no_sanitize_or_inline for the
> > > arch_ functions, because we really want them to be __always_inline
> > > (also to avoid calls to these functions in uaccess regions, which
> > > objtool would notice).
> > >
> > > I think the easiest way to resolve this is to wrap the accesses in
> > > __*_ONCE with data_race().
> >
> > But we can't... because I need arch_atomic_*() and __READ_ONCE() to not
> > call out to _ANYTHING_.
> >
> > Sadly, because the compilers are 'broken' that whole __no_sanitize thing
> > didn't work, but I'll be moving a whole bunch of code into .c files with
> > all the sanitizers killed dead. And we'll be validating it'll not be
> > calling out to anything.
>
> Hmm, I may have just run into this problem too. I'm using clang 11.0.1,
> but even if I do something like:
>
> unsigned long __no_sanitize_or_inline foo(unsigned long *p)
> {
>         return READ_ONCE_NOCHECK(*p);
> }
>
> then I /still/ get calls to __tcsan_func_{entry,exit} emitted by the
> compiler. Marco -- how do you turn this thing off?!

For Clang we have an option ("-mllvm
-tsan-instrument-func-entry-exit=0"), for GCC, I don't think we have
the option.

I had hoped we could keep these compiler changes optional for now, to
not require a very recent compiler. I'll send a patch to enable the
option, but keep it optional for now. Or do you think we require the
compiler to support this? Because then we'll only support Clang.

> I'm also not particularly fond of treating __{READ,WRITE}ONCE() as "atomic",
> since they're allowed to tear and I think callers should probably either be
> using data_race() explicitly or disabling instrumentation (assuming that's
> possible).

That point is fair enough. But how do we fix arch_atomic_{read,set} then?

Thanks,
-- Marco