[boost] Re: Re: recursive mutex and condition

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-11-03 11:56:59

"Vukasin Toroman" <vtoroman_at_[hidden]> wrote in message 
news:418892BF.7070701_at_astaro.de...
>> It looks like a documentation bug to me.
>
> in mho, if a bug exists than it is in design. if a recursive mutex 
> should not be used in a condition the condition should not accept 
> it.

They are intended to be used together.

> this is either a design flaw (allowing the condition to accept 
> recursive mutexes) or a implementation flaw (the condition should 
> only unlock the recursive mutex once instead all the way).

I don't think that makes sense, though:

   recursive_mutex m;
   condition c;
   bool b;

   void f()
   {
      lock lock1(m);

      //...

      lock lock2(m); //Recursive lock

      while (!b)
         c.wait(lock2); //***
   }

At the point marked "***", the thread calling c.wait() (call it thread 
#1) is waiting for another thread (thread #2) to wake it up. 
Presumably thread #2 will want to read or modify the resource 
protected by m before doing so (otherwise there's not much point in 
thread #1 waiting for it), but it won't be able to because the mutex 
is still locked by thread #1. If it tries to obtain the mutex, 
deadlock results.

Mike