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

Daniel P. Berrangé
2024-08-16T08:44:45Z

On Fri, Aug 16, 2024 at 07:44:28AM +0200, Thomas Huth wrote:
> On 15/08/2024 19.54, Peter Maydell wrote:
> > On Thu, 15 Aug 2024 at 12:05, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > >
> > > 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.
> >
> > Which is irritating if you want to be able to blanket say
> > '-Wno-silly-compiler-warning' and not see any of that
> > warning regardless of compiler version. That's why the
> > gcc behaviour is the way it is (i.e. -Wno-such-thingy
> > is neither a warning nor an error if it would be the only
> > warning/error), and if clang doesn't match it that's a shame.
>
> I thought that Clang would behave the same way as GCC, but apparently it
> does not (anymore?):

It is nothing new - clang has behaved this way wrt unknown warning flags
for as long as I remember.

With regards,
Daniel
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