Re: [boost] [thread] best practice to lock multiple mutexes

From: dizzy (dizzy_at_[hidden])
Date: 2008-02-01 08:27:30

On Friday 01 February 2008 14:21:40 Kowalke Oliver (QD IT PA AS) wrote:
> Hi,
> what's the best practice in following scenario:
> m threads and n resources (each resource is protected by a mutex)
> thread x needs read/write access to resources (...i,j,k,..)
> threads y,z,... do access another subset of resources at the same time
>
> inorder to protected deadlocks I would do following in the code of the
> different threads (code executed by the threads are different):
>
> ...
> try_lock: // label
> unique_lock< mutex > lk1( mtx1, try_to_lock_t);
> unique_lock< mutex > lk4( mtx2, try_to_lock_t);
> unique_lock< mutex > lk7( mtx3, try_to_lock_t);
> if ( ! ( lk1 && lk2 && lk3) ) goto try_lock;
> // all mutexes are locked
> // now modify safely resource 1,2,3
> ...
>
> Maybe you have a better pattern do solve this problem?

Im not sure if it solves exactly your problem but generally needing to lock 2 
mutexes for a given resource has the problem that it may very easily run into 
deadlock situations if another thread tries to lock the same mutexes in the 
reversed order. As such all you need to do is define an order relation across 
the mutexes (good choices for such an order is order the locks by the mutexes 
addresses in memory, this should provide you a complete strict order relation 
across all mutexes that may be used at some moment).

I'm not sure what your example is suposed to do, is it suposed to be a trylock 
or a normal lock (I supose that's what "try_to_lock_t" signals)? If it is a 
trylock then you just described a busy wait loop (will consume 100% CPU 
resources waiting for a lock). You just need to use normal blocking locks 
expect you should be careful about order of locks to be always the same for 
the same set of mutexes (as I described above). In my code (I needed often to 
lock 2 mutexes) I have made a lock wrapper class that does the ordered 
locking of the 2 mutexes.

-- 
Mihai RUSU					Email: dizzy_at_[hidden]
			"Linux is obsolete" -- AST