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 12:38:59 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	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>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.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 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

> ---
>  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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