Re: [PATCH] Remove unnecessary smp_wmb from clear_user_highpage()

[Oleg Nesterov <oleg@tv-sign.ru>]

(Off-topic)

Linus Torvalds wrote:
>
> A full lock/unlock *pair* should (as far as I know) always be equivalent 
> to a full memory barrier.

Is it so? I am not arguing, I am trying to understand.

> Because, by definition, no reads or writes 
> inside the locked region may escape outside it, and that in turn implies 
> that no access _outside_ the locked region may escape to the other side of 
> it.

This means that unlock + lock is a full barrier,

> However, neither a "lock" nor an "unlock" on *its*own* is a barrier at 
> all, at most they are semi-permeable barriers for some things, where 
> different architectures can be differently semi-permeable.

and this means that lock + unlock is not.

	A;
	lock();
	unlock();
	B;

If both A and B can leak into the critical section, they could be reordered
inside this section, so we can have

	lock();
	B;
	A;
	unlock();

Yes?

Oleg.

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