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

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2004-06-30 03:55:25

Michael Glassford <glassfordm_at_[hidden]> writes:

> Peter Dimov wrote:
>> Michael Glassford wrote:
>>
>>>Also, you could end up with some interesting situations like this:
>>>
>>>     void f(read_write_mutex m)
>>>     {
>>>         read_write_mutex::read_lock r(m);
>>>         if (...)
>>>         {
>>>             read_write_mutex::write_lock w(r); //lock promotion
>>>             //...
>>>         }
>>>         //Point A
>>>     }
>>>
>>>The most obvious implementation of promotion would be for the write lock
>>>to unlock the read lock if promotion succeeded, but leave it locked if
>>>promotion failed. But in the above code, this would mean that if
>>>promotion succeeds, neither lock will be locked at "Point A"; however
>>>if promotion fails, r will still be read-locked at point A.
>> Not necessarily, ~write_lock can (should) demote the lock back to read (or
>> whatever the initial condition of r was).
>
> True. But then, in the reverse case:
>
>       void f(read_write_mutex m)
>       {
>           read_write_mutex::write_lock w(m);
>           if (...)
>           {
>               read_write_mutex::read_lock r(w); //lock demotion
>               //...
>           }
>           //Point A
>       }
>
> It may not be possible for r's destructor to re-obtain the write lock

I don't think this usage should be allowed. Once you have a write lock (the
"higher class" of lock), then you keep your write lock until you release
it. If you want a read lock for the whole time, but only a write lock for a
portion of that time, then you write code like the first example. Your
"reverse case" actually fulfils that requirement, since there is a portion of
the code that only needs a read lock. Therefore it should look like:

       void f(read_write_mutex m)
       {
           read_write_mutex::read_lock r(m); // we need at least a read lock
                                             // for everything
           {
               read_write_mutex::write_lock w(r);
               // do stuff that needs the write lock here
           }
           if (...)
           {
               // we only need the read lock here
               //...
           }
           {
               read_write_mutex::write_lock w(r);
               // do more stuff that needs the write lock here
           }
       }

Anthony
-- 
Anthony Williams
Senior Software Engineer, Beran Instruments Ltd.