From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Hurley <peter@hurleysoftware.com>
Subject: Re: [PATCH] drm: Assert correct locking for
	drm_send_vblank_event
Date: Fri, 12 Sep 2014 13:42:59 -0400
Message-ID: <541330A3.4020108@hurleysoftware.com>
References: <1410529256-8602-1-git-send-email-daniel.vetter@ffwll.ch>
 <20140912152329.GI16043@nuc-i3427.alporthouse.com>
 <20140912153456.GH4740@phenom.ffwll.local>
 <20140912160442.GA14793@nuc-i3427.alporthouse.com>
 <54132777.5080804@hurleysoftware.com>
 <20140912172531.GI4740@phenom.ffwll.local>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <intel-gfx-bounces@lists.freedesktop.org>
In-Reply-To: <20140912172531.GI4740@phenom.ffwll.local>
List-Unsubscribe: <http://lists.freedesktop.org/mailman/options/intel-gfx>,
 <mailto:intel-gfx-request@lists.freedesktop.org?subject=unsubscribe>
List-Archive: <http://lists.freedesktop.org/archives/intel-gfx>
List-Post: <mailto:intel-gfx@lists.freedesktop.org>
List-Help: <mailto:intel-gfx-request@lists.freedesktop.org?subject=help>
List-Subscribe: <http://lists.freedesktop.org/mailman/listinfo/intel-gfx>,
 <mailto:intel-gfx-request@lists.freedesktop.org?subject=subscribe>
Errors-To: intel-gfx-bounces@lists.freedesktop.org
Sender: "Intel-gfx" <intel-gfx-bounces@lists.freedesktop.org>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>, Intel Graphics Development <intel-gfx@lists.freedesktop.org>, dri-devel@lists.freedesktop.org
List-Id: intel-gfx@lists.freedesktop.org

On 09/12/2014 01:25 PM, Daniel Vetter wrote:
> On Fri, Sep 12, 2014 at 01:03:51PM -0400, Peter Hurley wrote:
>> On 09/12/2014 12:04 PM, Chris Wilson wrote:
>>> On Fri, Sep 12, 2014 at 05:34:56PM +0200, Daniel Vetter wrote:
>>>> On Fri, Sep 12, 2014 at 04:23:29PM +0100, Chris Wilson wrote:
>>>>> On Fri, Sep 12, 2014 at 03:40:56PM +0200, Daniel Vetter wrote:
>>>>>> The comment says that the caller must hold the dev->event_lock
>>>>>> spinlock, so let's enforce this.
>>>>>>
>>>>>> A quick audit over all driver shows that except for the one place in
>>>>>> i915 which motivated this all callers fullfill this requirement
>>>>>> already.
>>>>>
>>>>> Replace the rogue WARN_ON_SMP(!spin_is_locked(&dev->event_lock)) in
>>>>> send_vblank_event() as well then.
>>>>
>>>> Meh, I've missed that one, that's actually better I think. I'll drop my
>>>> patch here.
>>>
>>> I thought assert_spin_lock was the preferred form?
>>
>> Actually, lockdep_assert_held() is the preferred form.
>>
>> See https://lkml.org/lkml/2014/9/3/171
> 
> Which unfortunately doesn't warn for all the normal users which are not
> insane enough to enable lockdep and so is totally useless to validate a
> driver that runs on metric piles of different chips (with a resulting
> combinatorial explosion of code-paths because hw designers are creative).
> And we rely a lot on random drive-by testers to report such issues.

I know. When I wrote [in that thread linked above]:

On Wed, Sep 03, 2014 at 10:50:01AM -0400, Peter Hurley wrote:
> So a lockdep-only assert is unlikely to draw attention to existing bugs,
> especially in established drivers.

here's the replies I got:

Peter Zijlstra <peterz@infradead.org> wrote:
> By the same logic lockdep will not find locking errors in established
> drivers.

and

On 09/04/2014 01:14 AM, Ingo Molnar wrote:
> Indeed, this patch is ill-advised in several ways:
> 
>   - it extends an API variant that we want to phase
> 
>   - emits a warning even if say lockdep has already emitted a
>     warning and locking state is not guaranteed to be consistent. 
> 
>   - makes the kernel more expensive once fully debugged, in that
>     non-fatal checks are unconditional.

:/

Regards,
Peter Hurley