Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference"

Daniel Vetter
2016-08-11T10:39:39Z

On Thu, Aug 11, 2016 at 11:50:21AM +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@intel.com>
>
> This reverts commit fa7d81bb3c269a2ee38b6e4d569d9eb8be1a78ad.
>
> As Peter explained:
> [...] lockless_dereference() is _stronger_ than READ_ONCE(), not weaker.
>
> [...]
>
> Also, clue is in the name: 'dereference', you don't actually dereference
> the pointer here, only load it.
>
> My next patch breaks compile on this, because it assumes you want to
> deference and thus also need the struct type visible (which it isn't
> here), so revert it.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

And ack-by: me for merging through whatever tree this makes sense for.
-Daniel

> ---
> drivers/gpu/drm/drm_fb_helper.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index ce54e985d91b..0a06f9120b5a 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ 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 (lockless_dereference(dev->master))
> + if (READ_ONCE(dev->master))
> return false;
>
> drm_for_each_crtc(crtc, dev) {
> --
> 2.8.1
>
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--
Daniel Vetter
Software Engineer, Intel Corporation