On Wednesday 15 August 2007 15:29:43 Arnd Bergmann wrote:

On Wednesday 15 August 2007, Paul E. McKenney wrote:

quoted

ACCESS_ONCE() is indeed intended to be used when actually loading or
storing the variable.  That said, I must admit that it is not clear to me
why you would want to add an extra order() rather than ACCESS_ONCE()ing
one or both of the adjacent accesses to that same variable.

So, what am I missing?

You're probably right, the only case I can construct is something like

	if (ACCESS_ONCE(x)) {
		...
		ACCESS_ONCE(x)++;
	}

which would be slightly less efficient than

	if (x)
		x++;
	order(x);

because in the first case, you need to do two ordered accesses
but only one in the second case. However, I can't think of a case
where this actually makes a noticable difference in real life.

How can this example actually get used in a sane and race-free
way? This would need locking around the whole if
statement. But locking is a barrier.

-- 
Greetings Michael.