[cpp-threads] Yet another visibility question

[Paul E. McKenney]

On Fri, Jan 12, 2007 at 09:03:50PM +0100, Alexander Terekhov wrote:
> On 1/12/07, Paul E. McKenney <paulmck at linux.vnet.ibm.com> wrote:
> [...]
> >> Think of implementing SC  with per-variable locks on a large x86/IA32
> >> NUMA configuration with multiple FSBs or whatever. It won't be any
> >> better than cmpxchg for loads and xchg for stores (both locked).
> >
> >My understanding is that locking is only guaranteed to force SC execution
> >of critical sections when all of the critical sections of interest
> >are guarded by the same lock.  Therefore, per-variable locks will not
> >necessarily force SC execution of all accesses.
> 
> Give an example (for x86/IA32).

I will start with a blatant example to make sure that we are really
talking about the same thing.  So, for a starting point using Linux-kernel
notation:

o	Shared data:

	DEFINE_MUTEX(mutex1);
	DEFINE_MUTEX(mutex2);
	int a = 0, b = 0, c = 0;

o	Thread 1:

	mutex_lock(&mutex1);
	a = 1;
	b = 1;
	mutex_unlock(&mutex1);

o	Thread 2:

	mutex_lock(&mutex2);
	c = 1;
	mutex_unlock(&mutex2);

The two critical sections do not exclude each other, so could overlap
arbitrarily.  In this example, you would actually be worse off with
locks than with per-variable cmpxchg.

Or am I missing your point?

							Thanx, Paul