On Wed, Jun 22, 2016 at 08:46:12AM +0100, Chris Wilson wrote:
> We are only documenting that the read is outside of the lock, and do not
> require strict ordering on the operation. In this case the more relaxed
> lockless_dereference() will suffice.
No, no, no... This is 'broken'. lockless_dereference() is _stronger_
than READ_ONCE(), not weaker.
lockless_dereference() is a wrapper around smp_read_barrier_depends()
and is used to form read dependencies. There is no read dependency here,
therefore using lockless_dereference() is entirely pointless.
Look at the definition of lockless_dereference(), it does a READ_ONCE()
and then smp_read_barrier_depends().
Also, clue is in the name: 'dereference', you don't actually dereference
the pointer here, only load it.
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -464,7 +464,7 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
>
> /* Sometimes user space wants everything disabled, so don't steal the
> * display if there's a master. */
> - if (READ_ONCE(dev->master))
> + if (lockless_dereference(dev->master))
> return false;
>
> drm_for_each_crtc(crtc, dev) {
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