Re: [PATCH] kernel: fix data race in put_pid

[Oleg Nesterov <oleg@redhat.com>]

On 09/18, Peter Zijlstra wrote:
>
> Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can
> more conveniently use atomics in control dependencies.
>
> Since we can assume atomic_read() implies a READ_ONCE(), we must only
> emit an extra smp_read_barrier_depends() in order to upgrade to
> READ_ONCE_CTRL() semantics.

...

> +static inline int atomic_read_ctrl(atomic_t *v)
> +{
> +	int val = atomic_read(v);
> +	smp_read_barrier_depends(); /* Enforce control dependency. */
> +	return val;
> +}

Help. I am starting to think that the control dependencies is even more
hard to understand that memory barriers...

So I assume that if we have

	int X = 0;
	atomic_t Y = ATOMIC_INIT(0);

	void w(void)
	{
		X = 1;
		atomic_inc_return(&Y);
	}

then

	void r(void)
	{
		if (atomic_read_ctrl(&Y))
			BUG_ON(X == 0);
	}

should be correct?  Why?

If not then I am even more confused.

Oleg.