[PATCH] drm: Assert correct locking for drm_send_vblank_event

[PATCH] drm: Assert correct locking for drm_send_vblank_event

[Daniel Vetter]

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.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_irq.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 80ff94ada75e..bf248eb9ffb2 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -907,6 +907,9 @@ void drm_send_vblank_event(struct drm_device *dev, int crtc,
 {
 	struct timeval now;
 	unsigned int seq;
+
+	assert_spin_locked(&dev->event_lock);
+
 	if (crtc >= 0) {
 		seq = drm_vblank_count_and_time(dev, crtc, &now);
 	} else {
-- 
1.9.3

Re: [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Chris Wilson]

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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Daniel Vetter]

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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [Intel-gfx] [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Chris Wilson]

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?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Peter Hurley]

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

Regards,
Peter Hurley

Re: [Intel-gfx] [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Daniel Vetter]

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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [PATCH] drm: Assert correct locking for drm_send_vblank_event

[Peter Hurley]

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