[cpp-threads] D2335 (sequential consistency proof) revision

[Howard Hinnant]

On Aug 24, 2007, at 5:29 PM, Boehm, Hans wrote:

> [Copied Doug explicitly, since I'd like his reaction, particularly  
> since
> it also suggests a corresponding change to JSR166.]
>
> Re: handling of trylock in the memory model:
>
>> From:  Lawrence Crowl
>> The reasoning is that if SC is a strong goal, then we need to
>> provide the mechanisms to make it achievable.  An alternate
>> approach is to disallow this use of the trylock.
>>
> Here's another possible way out of this.  I'm not 100% sure this is
> correct yet, but it seems like it on first glance:
>
> We allow trylock and timedlock to fail spuriously, even if the lock  
> was
> never held.
>
> The cause of all the problems with trylock and timedlock are that they
> see a "write" (lock) that didn't happen before it.  I think this  
> removes
> that problem, since a failed trylock can no longer be abused as a  
> read;
> it conveys no information.
>
> For any reasonable trylock-based code I have written or can think of,
> this doesn't matter.  If the trylock fails, the code either eventually
> retires, or proceeds in a way that's safe without the lock.
>
> This has no actual implementation impact, in that implementations that
> never fail spuriously are fine.  The programmer is just not allowed to
> rely on that behavior.  This is purely a way to characterize "bad"  
> uses
> of trylock to the programmer.
>
> If we do this, I believe we no longer need acquire semantics for a
> failed trylock.  I think that also applies to Java, which is why it
> might be an attractive change there.
>
> Would everyone be OK with this?

It sounds good to me.  I'm not an expert in the visibility area so I'd  
like to double check my understanding.

This code which takes two locks/mutexes and locks them without  
deadlock, in part by using try_lock, is not affected by Hans'  
proposal, right?

template <class L1, class L2>
void
lock(L1& l1, L2& l2)
{
     while (true)
     {
         l1.lock();
         if (l2.try_lock())
             break;
         l1.unlock();
         l2.lock();
         if (l1.try_lock())
             break;
         l2.unlock();
     }
}

-Howard