Re: [boost] Copyable mutex

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2007-07-23 03:53:08

"Peter Dimov" <pdimov_at_[hidden]> writes:

> Phillip Hellewell wrote:
>
>> However, even the simplest _assignable_ class seems almost impossible
>> to me to make thread-safe.  You can synchronize operator= using both
>> its own mutex(es) and rhs's mutex(es), which seems to solve the
>> problem.  But how do you prevent deadlock from something like this?
>>
>> Thread 1: x = y
>> Thread 2: y = x
>>
>> Thread 2 is going to lock the mutexes in the opposite order of thread
>> 1. Ouch!
>
> The classic solution is to lock the mutexes in ascending order of their 
> addresses, but I prefer
>
> X& X::operator=( X const& rhs )
> {
>     X tmp( rhs ); // temporarily locks rhs.mutex
>
>     scoped_lock lock( this->mutex );
>     swap( tmp ); // or move_from( tmp )
>
>     return *this;
> }
>
> It's always a good idea to only lock one mutex at a time if you can afford 
> it. 

I like the idea, but surely the publicly-visible swap should lock both
mutexes? Wouldn't this be better as internal_swap()?

Anthony
-- 
Anthony Williams
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