From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Vetter Subject: Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference" Date: Thu, 11 Aug 2016 12:38:59 +0200 Message-ID: <20160811103859.GL6232@phenom.ffwll.local> References: <1470909022-687-1-git-send-email-johannes@sipsolutions.net> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: Received: from mail-wm0-x242.google.com (mail-wm0-x242.google.com [IPv6:2a00:1450:400c:c09::242]) by gabe.freedesktop.org (Postfix) with ESMTPS id 54D206E939 for ; Thu, 11 Aug 2016 10:39:04 +0000 (UTC) Received: by mail-wm0-x242.google.com with SMTP id o80so571330wme.0 for ; Thu, 11 Aug 2016 03:39:04 -0700 (PDT) Content-Disposition: inline In-Reply-To: <1470909022-687-1-git-send-email-johannes@sipsolutions.net> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" To: Johannes Berg Cc: Johannes Berg , Peter Zijlstra , intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, Daniel Vetter , "Paul E . McKenney" , Ingo Molnar List-Id: dri-devel@lists.freedesktop.org T24gVGh1LCBBdWcgMTEsIDIwMTYgYXQgMTE6NTA6MjFBTSArMDIwMCwgSm9oYW5uZXMgQmVyZyB3 cm90ZToKPiBGcm9tOiBKb2hhbm5lcyBCZXJnIDxqb2hhbm5lcy5iZXJnQGludGVsLmNvbT4KPiAK PiBUaGlzIHJldmVydHMgY29tbWl0IGZhN2Q4MWJiM2MyNjlhMmVlMzhiNmU0ZDU2OWQ5ZWI4YmUx YTc4YWQuCj4gCj4gQXMgUGV0ZXIgZXhwbGFpbmVkOgo+ICAgWy4uLl0gbG9ja2xlc3NfZGVyZWZl cmVuY2UoKSBpcyBfc3Ryb25nZXJfIHRoYW4gUkVBRF9PTkNFKCksIG5vdCB3ZWFrZXIuCj4gCj4g ICBbLi4uXQo+IAo+ICAgQWxzbywgY2x1ZSBpcyBpbiB0aGUgbmFtZTogJ2RlcmVmZXJlbmNlJywg eW91IGRvbid0IGFjdHVhbGx5IGRlcmVmZXJlbmNlCj4gICB0aGUgcG9pbnRlciBoZXJlLCBvbmx5 IGxvYWQgaXQuCj4gCj4gTXkgbmV4dCBwYXRjaCBicmVha3MgY29tcGlsZSBvbiB0aGlzLCBiZWNh dXNlIGl0IGFzc3VtZXMgeW91IHdhbnQgdG8KPiBkZWZlcmVuY2UgYW5kIHRodXMgYWxzbyBuZWVk IHRoZSBzdHJ1Y3QgdHlwZSB2aXNpYmxlICh3aGljaCBpdCBpc24ndAo+IGhlcmUpLCBzbyByZXZl cnQgaXQuCj4gCj4gQ2M6IFBldGVyIFppamxzdHJhIDxwZXRlcnpAaW5mcmFkZWFkLm9yZz4KPiBT aWduZWQtb2ZmLWJ5OiBKb2hhbm5lcyBCZXJnIDxqb2hhbm5lcy5iZXJnQGludGVsLmNvbT4KClJl dmlld2VkLWJ5OiBEYW5pZWwgVmV0dGVyIDxkYW5pZWwudmV0dGVyQGZmd2xsLmNoPgoKQW5kIGFj ay1ieTogbWUgZm9yIG1lcmdpbmcgdGhyb3VnaCB3aGF0ZXZlciB0cmVlIHRoaXMgbWFrZXMgc2Vu c2UgZm9yLgotRGFuaWVsCgo+IC0tLQo+ICBkcml2ZXJzL2dwdS9kcm0vZHJtX2ZiX2hlbHBlci5j IHwgMiArLQo+ICAxIGZpbGUgY2hhbmdlZCwgMSBpbnNlcnRpb24oKyksIDEgZGVsZXRpb24oLSkK PiAKPiBkaWZmIC0tZ2l0IGEvZHJpdmVycy9ncHUvZHJtL2RybV9mYl9oZWxwZXIuYyBiL2RyaXZl cnMvZ3B1L2RybS9kcm1fZmJfaGVscGVyLmMKPiBpbmRleCBjZTU0ZTk4NWQ5MWIuLjBhMDZmOTEy MGI1YSAxMDA2NDQKPiAtLS0gYS9kcml2ZXJzL2dwdS9kcm0vZHJtX2ZiX2hlbHBlci5jCj4gKysr IGIvZHJpdmVycy9ncHUvZHJtL2RybV9mYl9oZWxwZXIuYwo+IEBAIC00NjQsNyArNDY0LDcgQEAg c3RhdGljIGJvb2wgZHJtX2ZiX2hlbHBlcl9pc19ib3VuZChzdHJ1Y3QgZHJtX2ZiX2hlbHBlciAq ZmJfaGVscGVyKQo+ICAKPiAgCS8qIFNvbWV0aW1lcyB1c2VyIHNwYWNlIHdhbnRzIGV2ZXJ5dGhp bmcgZGlzYWJsZWQsIHNvIGRvbid0IHN0ZWFsIHRoZQo+ICAJICogZGlzcGxheSBpZiB0aGVyZSdz IGEgbWFzdGVyLiAqLwo+IC0JaWYgKGxvY2tsZXNzX2RlcmVmZXJlbmNlKGRldi0+bWFzdGVyKSkK PiArCWlmIChSRUFEX09OQ0UoZGV2LT5tYXN0ZXIpKQo+ICAJCXJldHVybiBmYWxzZTsKPiAgCj4g IAlkcm1fZm9yX2VhY2hfY3J0YyhjcnRjLCBkZXYpIHsKPiAtLSAKPiAyLjguMQo+IAo+IF9fX19f X19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fCj4gZHJpLWRldmVsIG1h aWxpbmcgbGlzdAo+IGRyaS1kZXZlbEBsaXN0cy5mcmVlZGVza3RvcC5vcmcKPiBodHRwczovL2xp c3RzLmZyZWVkZXNrdG9wLm9yZy9tYWlsbWFuL2xpc3RpbmZvL2RyaS1kZXZlbAoKLS0gCkRhbmll bCBWZXR0ZXIKU29mdHdhcmUgRW5naW5lZXIsIEludGVsIENvcnBvcmF0aW9uCmh0dHA6Ly9ibG9n LmZmd2xsLmNoCl9fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19f CmRyaS1kZXZlbCBtYWlsaW5nIGxpc3QKZHJpLWRldmVsQGxpc3RzLmZyZWVkZXNrdG9wLm9yZwpo dHRwczovL2xpc3RzLmZyZWVkZXNrdG9wLm9yZy9tYWlsbWFuL2xpc3RpbmZvL2RyaS1kZXZlbAo= From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752297AbcHKKjj (ORCPT ); Thu, 11 Aug 2016 06:39:39 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:35574 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751062AbcHKKjh (ORCPT ); Thu, 11 Aug 2016 06:39:37 -0400 Date: Thu, 11 Aug 2016 12:38:59 +0200 From: Daniel Vetter To: Johannes Berg Cc: linux-kernel@vger.kernel.org, Johannes Berg , Peter Zijlstra , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Daniel Vetter , "Paul E . McKenney" , Ingo Molnar Subject: Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference" Message-ID: <20160811103859.GL6232@phenom.ffwll.local> Mail-Followup-To: Johannes Berg , linux-kernel@vger.kernel.org, Johannes Berg , Peter Zijlstra , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Daniel Vetter , "Paul E . McKenney" , Ingo Molnar References: <1470909022-687-1-git-send-email-johannes@sipsolutions.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1470909022-687-1-git-send-email-johannes@sipsolutions.net> X-Operating-System: Linux phenom 4.6.0-1-amd64 User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 11, 2016 at 11:50:21AM +0200, Johannes Berg wrote: > From: Johannes Berg > > 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 > Signed-off-by: Johannes Berg Reviewed-by: Daniel Vetter 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@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch