Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression

[Marco Elver]

On Wed, 27 May 2020 at 09:22, Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Wed, May 27, 2020 at 01:10:00AM +0200, Arnd Bergmann wrote:
> > On Tue, May 26, 2020 at 9:00 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > >
> > > On Tue, May 26, 2020 at 7:33 PM 'Marco Elver' via Clang Built Linux
> > > <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > > > On Tue, 26 May 2020, Marco Elver wrote:
> > > > > On Tue, 26 May 2020 at 14:19, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > > Note that an 'allyesconfig' selects KASAN and not KCSAN by default.
> > > > > But I think that's not relevant, since KCSAN-specific code was removed
> > > > > from ONCEs. In general though, it is entirely expected that we have a
> > > > > bit longer compile times when we have the instrumentation passes
> > > > > enabled.
> > > > >
> > > > > But as you pointed out, that's irrelevant, and the significant
> > > > > overhead is from parsing and pre-processing. FWIW, we can probably
> > > > > optimize Clang itself a bit:
> > > > > https://github.com/ClangBuiltLinux/linux/issues/1032#issuecomment-633712667
> > > >
> > > > Found that optimizing __unqual_scalar_typeof makes a noticeable
> > > > difference. We could use C11's _Generic if the compiler supports it (and
> > > > all supported versions of Clang certainly do).
> > > >
> > > > Could you verify if the below patch improves compile-times for you? E.g.
> > > > on fs/ocfs2/journal.c I was able to get ~40% compile-time speedup.
> > >
> > > Yes, that brings both the preprocessed size and the time to preprocess it
> > > with clang-11 back to where it is in mainline, and close to the speed with
> > > gcc-10 for this particular file.
> > >
> > > I also cross-checked with gcc-4.9 and gcc-10 and found that they do see
> > > the same increase in the preprocessor output, but it makes little difference
> > > for preprocessing performance on gcc.
> >
> > Just for reference, I've tested this against a patch I made that completely
> > shortcuts READ_ONCE() on anything but alpha (which needs the
> > read_barrier_depends()):
> >
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -224,18 +224,21 @@ void ftrace_likely_update(struct
> > ftrace_likely_data *f, int val,
> >   * atomicity or dependency ordering guarantees. Note that this may result
> >   * in tears!
> >   */
> > -#define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> > +#define __READ_ONCE(x) (*(const volatile typeof(x) *)&(x))
> >
> > +#ifdef CONFIG_ALPHA /* smp_read_barrier_depends is a NOP otherwise */
> >  #define __READ_ONCE_SCALAR(x)                                          \
> >  ({                                                                     \
> >         __unqual_scalar_typeof(x) __x = __READ_ONCE(x);                 \
> >         smp_read_barrier_depends();                                     \
> > -       (typeof(x))__x;                                                 \
> > +       __x;                                                            \
> >  })
> > +#else
> > +#define __READ_ONCE_SCALAR(x) __READ_ONCE(x)
> > +#endif
>
> Nice! FWIW, I'm planning to have Alpha override __READ_ONCE_SCALAR()
> eventually, so that smp_read_barrier_depends() can disappear forever. I
> just bit off more than I can chew for 5.8 :(
>
> However, '__unqual_scalar_typeof()' is still useful for
> load-acquire/store-release on arm64, so we still need a better solution to
> the build-time regression imo. I'm not fond of picking random C11 features
> to accomplish that, but I also don't have any better ideas...

We already use _Static_assert in the kernel, so it's not the first use
of a C11 feature.

> Is there any mileage in the clever trick from Rasmus?
>
> https://lore.kernel.org/r/6cbc8ae1-8eb1-a5a0-a584-2081fca1c4aa@xxxxxxxxxxxxxxxxxx

Apparently that one only works with GCC 7 or newer, and is only
properly defined behaviour since C11. It also relies on multiple
_Pragma. I'd probably take the arguably much cleaner _Generic solution
over that. ;-)

I think given that Peter and Arnd already did some testing, and it
works as intended, if you don't mind, I'll send a patch for the
_Generic version. At least that'll give us a more optimized
__unqual_scalar_typeof(). Any further optimizations to READ_ONCE()
like you mentioned then become a little less urgent.

Thanks,
-- Marco