[gcc r11-8638] libstdc++: Fix for deadlock in std::counting_semaphore [PR100806]

Thomas Rodgers rodgertq@gcc.gnu.org
Tue Jun 22 18:08:14 GMT 2021


https://gcc.gnu.org/g:c24384efbbaeeca3dbc53e7d1226a57bc16e9ad5

commit r11-8638-gc24384efbbaeeca3dbc53e7d1226a57bc16e9ad5
Author: Thomas Rodgers <rodgert@appliantology.com>
Date:   Tue Jun 22 10:59:07 2021 -0700

    libstdc++: Fix for deadlock in std::counting_semaphore [PR100806]
    
    libstdc++-v3/ChangeLog:
            PR libstdc++/100806
            * 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.
    
    (cherry picked from commit e02840c1a92abecd211ffaf05b28329bcb534583)

Diff:
---
 libstdc++-v3/include/bits/semaphore_base.h         |  4 +-
 .../testsuite/30_threads/semaphore/100806.cc       | 57 ++++++++++++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 4948f0fd0bc..c970fd66a36 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -254,7 +254,9 @@ _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..2fa2628a278
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/100806.cc
@@ -0,0 +1,57 @@
+// { 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 <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();
+	}
+      }
+}


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