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

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2004-07-09 16:43:55

On Jul 9, 2004, at 4:11 PM, Peter Dimov wrote:

>> rw_mutex m;
>>
>> void invitation_for_deadlock()
>> {
>>      read_lock r(m);
>>      // ok to read
>>      upgradable_read_lock ur(m);
>>      r.unlock();
>>      write_lock w(ur);
>>      // ok to write
>> }
>>
>> And the next thing we'll know is that the entire NE US is falling into
>> a blackout. ;-)
>>
>> It took me awhile to spot the deadlock in the above code.  Does anyone
>> else see the above code as scary?  (scary as in looks ok but isn't).
>> Or is the deadlock obvious to everyone else (and thus not scary)?
>
> Not scary, IMO, even if not obvious, because it always deadlocks, as 
> soon as
> you attempt the "idiom". You can't miss it.

I'm not following "always deadlocks".  If only one thread enters, at 
what point does it deadlock (assuming no other thread playing with m)?  
Experimentally running through this on my prototype implementation with 
a single thread isn't deadlocking.  But perhaps my prototype is buggy?

-Howard