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

Paul E. McKenney
2016-08-11T22:28:22Z

On Thu, Aug 11, 2016 at 12:38:59PM +0200, Daniel Vetter wrote:
> 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

Initial testing says that the change below must precede the change
to the definition of lockless_dereference(), so the two should go
together.

If my upcoming testing of the two changes together pans out, I will
give you a Tested-by -- I am guessing that you don't want to wait
until the next merge window for these changes.

Thanx, Paul

> > ---
> > 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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> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
>