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

[Will Deacon]

On Tue, May 26, 2020 at 12:42:16PM +0200, Arnd Bergmann wrote:
> On Thu, May 21, 2020 at 10:21 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, May 21, 2020 at 7:22 AM 'Marco Elver' via Clang Built Linux
> > <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > It appears that compilers have trouble with nested statement
> > > expressions. Therefore remove one level of statement expression nesting
> > > from the data_race() macro. This will help us avoid potential problems
> > > in future as its usage increases.
> > >
> > > Link: https://lkml.kernel.org/r/20200520221712.GA21166@xxxxxxx
> > > Acked-by: Will Deacon <will@xxxxxxxxxx>
> > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> >
> > Thanks Marco, I can confirm this series fixes the significant build
> > time regressions.
> >
> > Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> >
> > More measurements in: https://github.com/ClangBuiltLinux/linux/issues/1032
> >
> > Might want:
> > Reported-by: Borislav Petkov <bp@xxxxxxx>
> > Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > too.
> 
> I find this patch only solves half the problem: it's much faster than
> without the
> patch, but still much slower than the current mainline version. As far as I'm
> concerned, I think the build speed regression compared to mainline is not yet
> acceptable, and we should try harder.
> 
> I have not looked too deeply at it yet, but this is what I found from looking
> at a file in a randconfig build:
> 
> Configuration: see https://pastebin.com/raw/R9erCwNj

So this .config actually has KCSAN enabled. Do you still see the slowdown
with that disabled? Although not ideal, having a longer compiler time when
the compiler is being asked to perform instrumentation doesn't seem like a
show-stopper to me.

Will