reproducible bug in spinlock_ttas

[Dix Lorenz]

I managed to reproduce the hang I got in #254 :

int main(int /*argc*/, const char * /*argv*/ [])
{
   struct Item {
      int value{};
      Item(int v = 0) : value(v) {}
      Item(Item &&item)
      {
         boost::this_fiber::yield(); 
         value = item.value;
      }
      Item &operator=(Item &&item)
      {
         boost::this_fiber::yield();
         value = item.value;
         return *this;
      }
   };
   
   std::size_t channel_cap = 32;
   std::size_t producer_count = std::thread::hardware_concurrency();
   int max_value = 100;

   std::vector<std::thread>              threads;
   boost::fibers::buffered_channel<Item> channel(channel_cap);

   boost::fibers::barrier b(producer_count + 1);

   boost::fibers::fiber f_pop;

   unsigned sum = 0;

   for (unsigned i = 0; i < producer_count; ++i) {

      threads.emplace_back([&, i] {
         if (i == 0) {
            f_pop = boost::fibers::fiber([&] {
               {
                  Item x{};
                  while (boost::fibers::channel_op_status::success == channel.pop(x)) {
                     sum += x.value;
                  }
               }
            });
         }
         boost::fibers::fiber f([&] {
            for (int x = 0; x < max_value; ++x)
               channel.push(Item{ x });
         });
         f.join();
         b.wait();
         if (i == 0)
            f_pop.join();
      });
   }

   b.wait();
   channel.close();

   for (auto &t : threads)
      t.join();
      
   std::cout << sum << std::endl;

Obviously I am not really yielding the fiber in the move constructor/assignment, but when using buffered_channel::pop in a loop like this (which is perfectly reasonable) these get called; in my case the "previous" item returned an internal resource into a pool which was protected by a mutex which might yield the fiber and at that moment the channel is locked.

I think this could be fixed by spinlock_ttas::lock yielding this_fiber and not this_thread; I can't test that, this_fiber is not available in spinlock_ttas.hpp, I don't know how to yield the current fiber with the operations available in that header.

Technically it could probably also be solved in buffered_channel::pop(), but that would get really ugly.

At the very least this needs a warning in the documentation.

[Kapai]

Item &operator=(Item &&item) { boost::this_fiber::yield(); value = item.value; return *this; }
You overloaded the method.
channel.push internal use std::move.
internal spinlock_ttas is locked. After yielding, the internal lock is always locked.
I think this is normal, just need some hint text.