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

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Date:	Thu, 11 Aug 2016 11:26:47 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Johannes Berg <johannes@...solutions.net>,
	linux-kernel@...r.kernel.org,
	Johannes Berg <johannes.berg@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	Daniel Vetter <daniel.vetter@...el.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to
 lockless_dereference"

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@...el.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@...radead.org>
> > Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.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
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@...ts.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
> 

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