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

Jonathan Wakely jwakely.gcc@gmail.com
Thu Jun 17 16:45:59 GMT 2021


On Wed, 16 Jun 2021 at 20:53, Thomas Rodgers <rodgert@appliantology.com> wrote:
>
> 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.

OK for trunk and 11, thanks.


> ---
>  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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