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

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Message-Id: <1470909022-687-1-git-send-email-johannes@sipsolutions.net>
Date:	Thu, 11 Aug 2016 11:50:21 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...el.com>,
	dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
	Johannes Berg <johannes.berg@...el.com>
Subject: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference"

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>
---
 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