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

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On 8/15/24 10:54, Peter Maydell wrote:
On Thu, 15 Aug 2024 at 12:05, Daniel P. Berrangé <berrange@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
---
  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.


It's why I chose to implement this using cc.has_argument(), instead of trying to identify compiler/version.

thanks
-- PMM




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