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

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2004-07-09 18:33:39

On Jul 9, 2004, at 6:05 PM, Peter Dimov wrote:

> 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"? ;-)

<chuckle>  Only fools and idiots *never* change their mind! :-)

So that's two votes for scary.  I'm still undecided if it is "too 
scary".  I.e. will documentation make upgradable_read_lock sufficiently 
safe?  I'm hoping so, because there seems to be a vocal need for that 
functionality, and I don't see a good alternative at the moment.

A good rule of thumb for using rw_mutex seems to be that you should 
never hold more than one lock on it in the same thread.  Now that I 
write that though, I'm wondering if holding a recursive read_lock might 
be safe enough....  haven't looked into yet.  But holding a read_lock 
and an upgradable_read_lock in the same thread is definitely 
ill-advised, even if only for an instant.

At great extra expense one could put a runtime check into rw_mutex for 
one thread owning both a read_mutex and an upgradable_read_mutex.  It 
would require storing a vector<thread_id> for every thread sharing read 
ownership.  That would probably make the rw_mutex so expensive as to be 
useless though.

-Howard