GCC+TSan error

[alandefreitas]

Compiling Asio with GCC+TSan gives me the following error:

In function 'void std::atomic_thread_fence(memory_order)',
    inlined from 'boost::asio::detail::std_fenced_block::std_fenced_block(full_t)' at ./boost/asio/detail/std_fenced_block.hpp:46:29:
/usr/include/c++/12/bits/atomic_base.h:133:26: error: 'atomic_thread_fence' is not supported with '-fsanitize=thread' [-Werror=tsan]
  133 |   { __atomic_thread_fence(int(__m)); }
      |     ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
In function 'void std::atomic_thread_fence(memory_order)',
    inlined from 'boost::asio::detail::std_fenced_block::~std_fenced_block()' at ./boost/asio/detail/std_fenced_block.hpp:52:29:
/usr/include/c++/12/bits/atomic_base.h:133:26: error: 'atomic_thread_fence' is not supported with '-fsanitize=thread' [-Werror=tsan]
  133 |   { __atomic_thread_fence(int(__m)); }
      |     ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
cc1plus: all warnings being treated as errors

Because TSan does not support atomic_thread_fence.

LLVM's https://github.com/llvm/llvm-project/blob/ad1161f9b5ff251b80788033e4db82b5a11b187b/compiler-rt/include/sanitizer/tsan_interface.h#L24-L25 can be used instead.

I'm not sure there's anything actionable about this but I'm just reporting it.

I've also noticed BOOST_ASIO_DISABLE_FENCED_BLOCK is there. I imagine disabling the fenced block takes care of the GCC warning, but if these fences are really necessary, TSan will report data races.

[natale-p]

There is any news on this? Thanks

[d3matt]

I could open a PR with a patch that looks something like this below. I've only tested it so far with gcc14 on Linux x86_64, but it makes TSAN happy in my code that uses the boost version of this.

diff --git a/include/asio/detail/std_fenced_block.hpp b/include/asio/detail/std_fenced_block.hpp
index 2d79656be..cf8619d04 100644
--- a/include/asio/detail/std_fenced_block.hpp
+++ b/include/asio/detail/std_fenced_block.hpp
@@ -21,6 +21,17 @@
 
 #include "asio/detail/push_options.hpp"
 
+#if defined(__has_feature)
+# if __has_feature(thread_sanitizer)
+#  include "sanitizer/tsan_interface.h"
+#  define USE_TSAN_SEMANTICS 1
+# else
+#  define USE_TSAN_SEMANTICS 0
+# endif
+#else
+# define USE_TSAN_SEMANTICS 0
+#endif
+
 namespace asio {
 namespace detail {
 
@@ -39,14 +50,27 @@ public:
   // Constructor for a full fenced block.
   explicit std_fenced_block(full_t)
   {
+#if USE_TSAN_SEMANTICS
+    __tsan_acquire(&tsan);
+#else
     std::atomic_thread_fence(std::memory_order_acquire);
+#endif
   }
 
   // Destructor.
   ~std_fenced_block()
   {
+#if USE_TSAN_SEMANTICS
+    __tsan_release(&tsan);
+#else
     std::atomic_thread_fence(std::memory_order_release);
+#endif
   }
+
+#if USE_TSAN_SEMANTICS
+private:
+    int tsan;
+#endif
 };
 
 } // namespace detail