Re: [PATCH v2 1/4] meson: hide tsan related warnings

Daniel P. Berrangé
2024-08-15T11:05:36Z

On Thu, Aug 15, 2024 at 11:12:39AM +0100, Peter Maydell wrote:
> On Wed, 14 Aug 2024 at 23:42, Pierrick Bouvier
> <pierrick.bouvier@linaro.org> wrote:
> >
> > When building with gcc-12 -fsanitize=thread, gcc reports some
> > constructions not supported with tsan.
> > Found on debian stable.
> >
> > qemu/include/qemu/atomic.h:36:52: error: ‘atomic_thread_fence’ is not supported with ‘-fsanitize=thread’ [-Werror=tsan]
> > 36 | #define smp_mb() ({ barrier(); __atomic_thread_fence(__ATOMIC_SEQ_CST); })
> > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> > ---
> > meson.build | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/meson.build b/meson.build
> > index 81ecd4bae7c..52e5aa95cc0 100644
> > --- a/meson.build
> > +++ b/meson.build
> > @@ -499,7 +499,15 @@ if get_option('tsan')
> > prefix: '#include <sanitizer/tsan_interface.h>')
> > error('Cannot enable TSAN due to missing fiber annotation interface')
> > endif
> > - qemu_cflags = ['-fsanitize=thread'] + qemu_cflags
> > + tsan_warn_suppress = []
> > + # gcc (>=11) will report constructions not supported by tsan:
> > + # "error: ‘atomic_thread_fence’ is not supported with ‘-fsanitize=thread’"
> > + # https://gcc.gnu.org/gcc-11/changes.html
> > + # However, clang does not support this warning and this triggers an error.
> > + if cc.has_argument('-Wno-tsan')
> > + tsan_warn_suppress = ['-Wno-tsan']
> > + endif
>
> That last part sounds like a clang bug -- -Wno-foo is supposed
> to not be an error on compilers that don't implement -Wfoo for
> any value of foo (unless some other warning/error would also
> be emitted).

-Wno-foo isn't an error, but it is a warning... which we then
turn into an error due to -Werror, unless we pass -Wno-unknown-warning-option
to clang.

> At any rate, that's how gcc does it
> (see the paragraph "When an unrecognized warning option ..."
> in https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html )
> and I thought clang did too...
>
> thanks
> -- PMM
>

With regards,
Daniel
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