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

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-06-30 09:33:50

Peter Dimov wrote:
> Michael Glassford wrote:
> 
>>Finally, to take two more realistic use cases:
>>
>>      void f(read_write_mutex m)
>>      {
>>          read_write_mutex::write_lock w(m);
>>
>>          //...
>>
>>          read_write_mutex::read_lock r(w); //lock promotion
>>
>>         //...
>>      }
>>
>>And:
>>
>>      void f(read_write_mutex m)
>>      {
>>          read_write_mutex::write_lock r(m);
>>
>>          //...
>>
>>          read_write_mutex::read_lock w(r); //lock demotion
>>
>>         //...
>>      }
>>
>>The second lock does unnecessary work in demoting/promoting the lock
>>for the first lock, which then immediately unlocks it.
> 
> 
> This is a very good point, since in the second example the unnecessary
> promotion would block. 

One thing that I haven't made clear is that (so far at least) the 
Boost.Threads read/write mutex doesn't support infinitely-blocking 
promotion, which has problems (e.g. if two threads with read locks block 
waiting to be promoted to write locks, they deadlock); it only supports 
try_promote() and timed_promote(). So there shouldn't be a problem with 
blocking, at least, in the second example.

 > This seems to suggest that read_write_lock is
> necessary in such a scenario.

Looks like I really messed up my examples; I'm glad you could see my
point in spite of that. Of course, what I meant was this:

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

           //...

           read_write_mutex::write_lock w(r); //lock promotion

          //...
       }

And:

       void f(read_write_mutex m)
       {
           read_write_mutex::write_lock w(m);

           //...

           read_write_mutex::read_lock r(w); //lock demotion

          //...
       }

Mike