Re: [boost] Re: Boost.Threads: Do we need all those mutexes?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-05 15:33:51

Howard Hinnant wrote:
> On Jun 29, 2004, at 12:30 PM, Michael Glassford wrote:
>
>> Also, you could end up with some interesting situations like this:

[...]

Let's assume that the guarded entity is

int x;

>>     void f(read_write_mutex m)
>>     {
>>         read_write_mutex::read_lock r(m);

            int y = x;

>>         if (...)
>>         {
>>             read_write_mutex::write_lock w(r); //lock promotion

            assert( x == y); // always holds, we had a read+ lock all the
time

>>             //...
>>         }
>>         //Point A
>>     }
>
> How 'bout:
>
>      void f(read_write_mutex& m)
>      {
>          read_write_mutex::read_lock r(m);

            int y = x;

>          if (...)
>          {
>              r.unlock();

Thread switch, write lock, write to x happens here.

>              read_write_mutex::write_lock w(m);
>              //...  m is write locked

            assert( x == y); // may fail

>              w.transfer_to_read_lock(r);
>          }
>          //Point A ... m is read locked
>      }