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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-09 17:05:03

Howard Hinnant wrote:
> 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?

I wouldn't bet on that. Can I change my opinion to "scary"? ;-)