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

[Thomas Rodgers]

Tested x86_64-pc-linux-gnu.
Committed to master, backported to releases/gcc-11.

On Thu, Jun 17, 2021 at 9:46 AM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

> 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
> >
>
>