[PATCH] libstdc++: Fix for deadlock in std::counting_semaphore [PR100806]

Thomas Rodgers rodgert@appliantology.com
Wed Jun 16 19:52:18 GMT 2021


Same as previous version except removing the copyright notice from the
test.

libstdc++-v3/ChangeLog:
	libstdc++/PR100806
	* include/bits/semaphore_base.h (__atomic_semaphore::_M_release():
	Force _M_release() to wake all waiting threads.
	* testsuite/30_threads/semaphore/100806.cc: New test.
---
 libstdc++-v3/include/bits/semaphore_base.h    |  4 +-
 .../testsuite/30_threads/semaphore/100806.cc  | 60 +++++++++++++++++++
 2 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/100806.cc

diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 9a55978068f..c4565d7e560 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -256,7 +256,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       if (__update > 1)
 	__atomic_notify_address_bare(&_M_counter, true);
       else
-	__atomic_notify_address_bare(&_M_counter, false);
+	__atomic_notify_address_bare(&_M_counter, true);
+// FIXME - Figure out why this does not wake a waiting thread
+//	__atomic_notify_address_bare(&_M_counter, false);
     }
 
   private:
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/100806.cc b/libstdc++-v3/testsuite/30_threads/semaphore/100806.cc
new file mode 100644
index 00000000000..938c2793be1
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/100806.cc
@@ -0,0 +1,60 @@
+// { dg-options "-std=gnu++2a -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+// { dg-add-options libatomic }
+
+#include <array>
+#include <iostream>
+#include <sstream>
+
+#include <thread>
+#include <semaphore>
+#include <mutex>
+#include <chrono>
+#include <vector>
+
+std::counting_semaphore<4> semaphore{6};
+
+std::mutex mtx;
+std::vector<std::string> results; 
+
+void thread_main(size_t x)
+{
+  semaphore.acquire();
+  std::this_thread::sleep_for(std::chrono::milliseconds(100));
+  semaphore.release();
+  {
+    std::ostringstream stm;
+    stm << "Thread " << x << " finished.";
+    std::lock_guard g{ mtx };
+    results.push_back(stm.str());
+  }
+}
+
+int main()
+{
+    
+    constexpr auto nthreads = 10;
+
+    std::vector<std::thread> threads(nthreads);
+
+
+    size_t counter{0};
+    for(auto& t : threads)
+    {
+        t = std::thread(thread_main, counter++);
+    }
+
+    for(auto& t : threads)
+      {
+        t.join();
+        {
+          std::lock_guard g{ mtx };
+          for (auto&& r : results)
+            std::cout << r << '\n';
+          std::cout.flush();
+          results.clear();
+        }
+      }
+}
-- 
2.26.2



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