Re: [PATCH v2 5/5] drm/msm: subclass work object for vblank events

[Jeykumar Sankaran <jsanka-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 2018-12-03 06:21, Sean Paul wrote:
> On Fri, Nov 30, 2018 at 04:21:15PM -0800, Jeykumar Sankaran wrote:
>> On 2018-11-30 12:07, Sean Paul wrote:
>> > On Fri, Nov 30, 2018 at 11:45:55AM -0800, Jeykumar Sankaran wrote:
>> > > On 2018-11-29 14:15, Sean Paul wrote:
>> > > > On Tue, Nov 20, 2018 at 02:04:14PM -0800, Jeykumar Sankaran wrote:
>> > > > > On 2018-11-07 07:55, Sean Paul wrote:
>> > > > > > On Tue, Nov 06, 2018 at 02:36:30PM -0800, Jeykumar Sankaran
> wrote:
>> > > > > > > msm maintains a separate structure to define vblank
>> > > > > > > work definitions and a list to track events submitted
>> > > > > > > to the workqueue. We can avoid this redundant list
>> > > > > > > and its protection mechanism, if we subclass the
>> > > > > > > work object to encapsulate vblank event parameters.
>> > > > > > >
>> > > > > > > changes in v2:
>> > > > > > > 	- subclass optimization on system wq (Sean Paul)
>> > > > > >
>> > > > > > I wouldn't do it like this, tbh. One problem is that you've
> lost
>> > your
>> > > > > > flush() on
>> > > > > > unbind, so there's no way to know if you have workers in the
> wild
>> > > > > > waiting
>> > > > > > to
>> > > > > > enable/disable vblank.
>> > > > > >
>> > > > > > Another issues is that AFAICT, we don't need a queue of
>> > > > > > enables/disables,
>> > > > > > but
>> > > > > > rather just the last requested state (ie: should we be on or
> off).
>> > So
>> > > > > > things
>> > > > > > don't need to be this complicated (and we're possibly
> thrashing
>> > vblank
>> > > > > > on/off
>> > > > > > for no reason).
>> > > > > >
>> > > > > > I'm still of the mind that you should just make this
> synchronous
>> > and
>> > > > be
>> > > > > > done
>> > > > > > with the threads (especially since we're still
>> > uncovering/introducing
>> > > > > > races!).
>> > > > > >
>> > > > > While scoping out the effort to make vblank events synchronous,
> I
>> > > > > found
>> > > > > that the spinlock locking order of vblank request sequence and
>> > vblank
>> > > > > callback
>> > > > > sequences are the opposite.
>> > > > >
>> > > > > In DPU, drm_vblank_enable acquires vblank_time_lock before
>> > registering
>> > > > > the crtc to encoder which happens after acquiring
> encoder_spinlock.
>> > > > > But
>> > > > > the vblank_callback acquires encoder_spinlock before accessing
> the
>> > > > > registered
>> > > > > crtc and calling into drm_vblank_handler which tries to acquire
>> > > > > vblank_time_lock.
>> > > > > Acquiring both vblank_time_lock and encoder_spinlock in the same
>> > > > > thread
>> > > > > is leading to deadlock.
>> > > >
>> > > > Hmm, I'm not sure I follow. Are you seeing issues where irq
> overlaps
>> > > > with
>> > > > enable/disable? I hacked in sync vblank enable/disable quickly to
> see
>> > if
>> > > > I
>> > > > could
>> > > > reproduce what you're seeing, but things seemed well behaved.
>> > > >
>> > >
>> > > The race is between drm_vblank_get/put and vblank_handler contexts.
>> > >
>> > > When made synchronous:
>> > >
>> > > while calling drm_vblank_get, the callstack looks like below:
>> > > drm_vblank_get -> drm_vblank_enable (acquires vblank_time_lock) ->
>> > > __enable_vblank -> dpu_crtc_vblank ->
>> > > dpu_encoder_toggle_vblank_for_crtc
>> > > (tries to acquire enc_spinlock)
>> > >
>> > > In vblank handler, the call stack will be:
>> > > dpu_encoder_phys_vid_vblank_irq -> dpu_encoder_vblank_callback
>> > > (acquires
>> > > enc_spinlock) -> dpu_crtc_vblank_callback -> drm_handle_vblank
>> > > (tries to
>> > > acquire vblank_time_lock)
>> >
>> > Hmm, I'm not sure how this can happen. We acquire and release the
>> > enc_spinlock
>> > before enabling the irq, yes we will hold on to the vbl_time_lock, but
>> > we
>> > shouldn't be trying to reacquire an encoder's spinlock after we've
>> > enabled
>> > it.
>> In the synchronous approach dpu_encoder_toggle_vblank_for_crtc(which
>> acquires the enc_spinlock) will be called while we
>> are holding the vbl_time_lock.
>> 
>> > I don't know how that can deadlock, since we should never be running
>> > enable and
>> > the handler concurrently.
>> >
>> I agree that vblank_irq handler should not be running before the 
>> enable
>> sequence. But
>> don't you expect the handler to be running while calling the
> vblank_disable
>> sequence?
> 
> This is an entirely different problem though. It's also one that is 
> easier
> to
> fix. I think we could probably grab the enc_spinlock in disable and 
> clear
> the
> crtc pointer.
> 
we do hold enc_spinlock in dpu_encoder_assign_crtc (drm/msm: dpu: Remove 
vblank_callback from encoder)
where we clear the crtc pointer.

> What I'm getting at is that there's no fundamental reason why we need 
> to
> have
> async vblank enable/disable.
> 
> Sean
> 
There is really no *need* to have them async. But I believe the reason 
why they
are implemented this way is to avoid deadlock between the below two 
paths.

Restating the above findings:
vblank_handlers and vblank enable/disable can run concurrently. The 
first trying to acquire
vbl_time_lock holding enc_spinlock. Other trying to acquire enc_spinlock 
holding
vbl_time_lock.

Thanks,
Jeykumar S.


>> vbl disable will try to acquire the locks in the opposite order to 
>> that
> of
>> irq_handler and the
>> same issue is bound to happen.
>> 
>> With your patch, you should be able to simulate this deadlock if you 
>> can
>> inject a delay
>> by adding a pr_err log in vblank_ctrl_queue_work
>> 
>> Thanks,
>> Jeykumar S.
>> 
>> > The only thing I can think of is that the vblank interrupts are firing
>> > after
>> > vblank has been disabled? In that case, it seems like we should
> properly
>> > flush
>> > them.
>> >
>> > Sean
>> >
>> >
>> > >
>> > >
>> > > > I do see that there is a chance to call drm_handle_vblank() while
>> > > > holding
>> > > > enc_spinlock, but couldn't find any obvious lock recursion there.
>> > > >
>> > > > Maybe a callstack or lockdep splat would help?
>> > > >
>> > > > Sean
>> > > >
>> > > >
>> > > > Here's my hack to bypass the display thread:
>> > > >
>> > > > diff --git a/drivers/gpu/drm/msm/msm_drv.c
>> > > > b/drivers/gpu/drm/msm/msm_drv.c
>> > > > index 9c9f7ff6960b38..5a3cac5825319e 100644
>> > > > --- a/drivers/gpu/drm/msm/msm_drv.c
>> > > > +++ b/drivers/gpu/drm/msm/msm_drv.c
>> > > > @@ -242,24 +242,19 @@ static void vblank_ctrl_worker(struct
>> > kthread_work
>> > > > *work)
>> > > >  static int vblank_ctrl_queue_work(struct msm_drm_private *priv,
>> > > >  					int crtc_id, bool enable)
>> > > >  {
>> > > > +	struct msm_kms *kms = priv->kms;
>> > > >  	struct msm_vblank_ctrl *vbl_ctrl = &priv->vblank_ctrl;
>> > > > -	struct vblank_event *vbl_ev;
>> > > >  	unsigned long flags;
>> > > >
>> > > > -	vbl_ev = kzalloc(sizeof(*vbl_ev), GFP_ATOMIC);
>> > > > -	if (!vbl_ev)
>> > > > -		return -ENOMEM;
>> > > > +	spin_lock_irqsave(&vbl_ctrl->lock, flags);
>> > > >
>> > > > -	vbl_ev->crtc_id = crtc_id;
>> > > > -	vbl_ev->enable = enable;
>> > > > +	if (enable)
>> > > > +		kms->funcs->enable_vblank(kms,
> priv->crtcs[crtc_id]);
>> > > > +	else
>> > > > +		kms->funcs->disable_vblank(kms,
> priv->crtcs[crtc_id]);
>> > > >
>> > > > -	spin_lock_irqsave(&vbl_ctrl->lock, flags);
>> > > > -	list_add_tail(&vbl_ev->node, &vbl_ctrl->event_list);
>> > > >  	spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
>> > > >
>> > > > -	kthread_queue_work(&priv->disp_thread[crtc_id].worker,
>> > > > -			&vbl_ctrl->work);
>> > > > -
>> > > >  	return 0;
>> > > >  }
>> > > >
>> > > Even with your patch above, I see frame is getting stuck but it
>> > > recovers
>> > in
>> > > a while.
>> > > The patch I tried was assigning
>> > crtc->funcs->enable_vblank/disable_vblank so
>> > > that
>> > > __enable_vblank can call crtc directly. But the above callstack is
>> > > still
>> > > valid for your patch.
>> > >
>> > > Thanks,
>> > > Jeykumar S.
>> > > >
>> > > >
>> > > > >
>> > > > > In MDP5, I see the same pattern between vblank_time_lock and
>> > list_lock
>> > > > which
>> > > > > is used to track the irq handlers.
>> > > > >
>> > > > > I believe that explains why msm_drv is queuing the vblank
>> > > > > enable/disable
>> > > > > works to WQ after acquiring vblank_time_lock.
>> > > > >
>> > > > > Thanks,
>> > > > > Jeykumar S.
>> > > > >
>> > > > > > Sean
>> > > > > >
>> > > > > > >
>> > > > > > > Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
>> > > > > > > ---
>> > > > > > >  drivers/gpu/drm/msm/msm_drv.c | 67
>> > > > > > +++++++++++++------------------------------
>> > > > > > >  drivers/gpu/drm/msm/msm_drv.h |  7 -----
>> > > > > > >  2 files changed, 20 insertions(+), 54 deletions(-)
>> > > > > > >
>> > > > > > > diff --git a/drivers/gpu/drm/msm/msm_drv.c
>> > > > > > b/drivers/gpu/drm/msm/msm_drv.c
>> > > > > > > index 6d6c73b..8da5be2 100644
>> > > > > > > --- a/drivers/gpu/drm/msm/msm_drv.c
>> > > > > > > +++ b/drivers/gpu/drm/msm/msm_drv.c
>> > > > > > > @@ -203,61 +203,44 @@ u32 msm_readl(const void __iomem
> *addr)
>> > > > > > >  	return val;
>> > > > > > >  }
>> > > > > > >
>> > > > > > > -struct vblank_event {
>> > > > > > > -	struct list_head node;
>> > > > > > > +struct msm_vblank_work {
>> > > > > > > +	struct work_struct work;
>> > > > > > >  	int crtc_id;
>> > > > > > >  	bool enable;
>> > > > > > > +	struct msm_drm_private *priv;
>> > > > > > >  };
>> > > > > > >
>> > > > > > >  static void vblank_ctrl_worker(struct work_struct *work)
>> > > > > > >  {
>> > > > > > > -	struct msm_vblank_ctrl *vbl_ctrl = container_of(work,
>> > > > > > > -						struct
>> > msm_vblank_ctrl,
>> > > > > > work);
>> > > > > > > -	struct msm_drm_private *priv = container_of(vbl_ctrl,
>> > > > > > > -					struct msm_drm_private,
>> > > > > > vblank_ctrl);
>> > > > > > > +	struct msm_vblank_work *vbl_work = container_of(work,
>> > > > > > > +						struct
>> > msm_vblank_work,
>> > > > > > work);
>> > > > > > > +	struct msm_drm_private *priv = vbl_work->priv;
>> > > > > > >  	struct msm_kms *kms = priv->kms;
>> > > > > > > -	struct vblank_event *vbl_ev, *tmp;
>> > > > > > > -	unsigned long flags;
>> > > > > > > -
>> > > > > > > -	spin_lock_irqsave(&vbl_ctrl->lock, flags);
>> > > > > > > -	list_for_each_entry_safe(vbl_ev, tmp,
>> > &vbl_ctrl->event_list, node)
>> > > > > > {
>> > > > > > > -		list_del(&vbl_ev->node);
>> > > > > > > -		spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
>> > > > > > > -
>> > > > > > > -		if (vbl_ev->enable)
>> > > > > > > -			kms->funcs->enable_vblank(kms,
>> > > > > > > -
>> > > > > > priv->crtcs[vbl_ev->crtc_id]);
>> > > > > > > -		else
>> > > > > > > -			kms->funcs->disable_vblank(kms,
>> > > > > > > -
>> > > > > > priv->crtcs[vbl_ev->crtc_id]);
>> > > > > > >
>> > > > > > > -		kfree(vbl_ev);
>> > > > > > > -
>> > > > > > > -		spin_lock_irqsave(&vbl_ctrl->lock, flags);
>> > > > > > > -	}
>> > > > > > > +	if (vbl_work->enable)
>> > > > > > > +		kms->funcs->enable_vblank(kms,
>> > > > > > priv->crtcs[vbl_work->crtc_id]);
>> > > > > > > +	else
>> > > > > > > +		kms->funcs->disable_vblank(kms,
>> > > > > > priv->crtcs[vbl_work->crtc_id]);
>> > > > > > >
>> > > > > > > -	spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
>> > > > > > > +	kfree(vbl_work);
>> > > > > > >  }
>> > > > > > >
>> > > > > > >  static int vblank_ctrl_queue_work(struct msm_drm_private
> *priv,
>> > > > > > >  					int crtc_id, bool enable)
>> > > > > > >  {
>> > > > > > > -	struct msm_vblank_ctrl *vbl_ctrl = &priv->vblank_ctrl;
>> > > > > > > -	struct vblank_event *vbl_ev;
>> > > > > > > -	unsigned long flags;
>> > > > > > > +	struct msm_vblank_work *vbl_work;
>> > > > > > >
>> > > > > > > -	vbl_ev = kzalloc(sizeof(*vbl_ev), GFP_ATOMIC);
>> > > > > > > -	if (!vbl_ev)
>> > > > > > > +	vbl_work = kzalloc(sizeof(*vbl_work), GFP_ATOMIC);
>> > > > > > > +	if (!vbl_work)
>> > > > > > >  		return -ENOMEM;
>> > > > > > >
>> > > > > > > -	vbl_ev->crtc_id = crtc_id;
>> > > > > > > -	vbl_ev->enable = enable;
>> > > > > > > +	INIT_WORK(&vbl_work->work, vblank_ctrl_worker);
>> > > > > > >
>> > > > > > > -	spin_lock_irqsave(&vbl_ctrl->lock, flags);
>> > > > > > > -	list_add_tail(&vbl_ev->node, &vbl_ctrl->event_list);
>> > > > > > > -	spin_unlock_irqrestore(&vbl_ctrl->lock, flags);
>> > > > > > > +	vbl_work->crtc_id = crtc_id;
>> > > > > > > +	vbl_work->enable = enable;
>> > > > > > > +	vbl_work->priv = priv;
>> > > > > > >
>> > > > > > > -	schedule_work(&vbl_ctrl->work);
>> > > > > > > +	schedule_work(&vbl_work->work);
>> > > > > > >
>> > > > > > >  	return 0;
>> > > > > > >  }
>> > > > > > > @@ -269,14 +252,13 @@ static int msm_drm_uninit(struct
> device
>> > *dev)
>> > > > > > >  	struct msm_drm_private *priv = ddev->dev_private;
>> > > > > > >  	struct msm_kms *kms = priv->kms;
>> > > > > > >  	struct msm_mdss *mdss = priv->mdss;
>> > > > > > > -	struct msm_vblank_ctrl *vbl_ctrl = &priv->vblank_ctrl;
>> > > > > > > -	struct vblank_event *vbl_ev, *tmp;
>> > > > > > >  	int i;
>> > > > > > >
>> > > > > > >  	/* We must cancel and cleanup any pending vblank
>> > enable/disable
>> > > > > > >  	 * work before drm_irq_uninstall() to avoid work
>> > re-enabling an
>> > > > > > >  	 * irq after uninstall has disabled it.
>> > > > > > >  	 */
>> > > > > > > +
>> > > > > > >  	msm_gem_shrinker_cleanup(ddev);
>> > > > > > >
>> > > > > > >  	drm_kms_helper_poll_fini(ddev);
>> > > > > > > @@ -292,12 +274,6 @@ static int msm_drm_uninit(struct device
>> > *dev)
>> > > > > > >  #endif
>> > > > > > >  	drm_mode_config_cleanup(ddev);
>> > > > > > >
>> > > > > > > -	flush_work(&vbl_ctrl->work);
>> > > > > > > -	list_for_each_entry_safe(vbl_ev, tmp,
>> > &vbl_ctrl->event_list, node)
>> > > > > > {
>> > > > > > > -		list_del(&vbl_ev->node);
>> > > > > > > -		kfree(vbl_ev);
>> > > > > > > -	}
>> > > > > > > -
>> > > > > > >  	/* clean up event worker threads */
>> > > > > > >  	for (i = 0; i < priv->num_crtcs; i++) {
>> > > > > > >  		if (priv->event_thread[i].thread) {
>> > > > > > > @@ -469,9 +445,6 @@ static int msm_drm_init(struct device
> *dev,
>> > > > struct
>> > > > > > drm_driver *drv)
>> > > > > > >  	priv->wq = alloc_ordered_workqueue("msm", 0);
>> > > > > > >
>> > > > > > >  	INIT_LIST_HEAD(&priv->inactive_list);
>> > > > > > > -	INIT_LIST_HEAD(&priv->vblank_ctrl.event_list);
>> > > > > > > -	INIT_WORK(&priv->vblank_ctrl.work, vblank_ctrl_worker);
>> > > > > > > -	spin_lock_init(&priv->vblank_ctrl.lock);
>> > > > > > >
>> > > > > > >  	drm_mode_config_init(ddev);
>> > > > > > >
>> > > > > > > diff --git a/drivers/gpu/drm/msm/msm_drv.h
>> > > > > > b/drivers/gpu/drm/msm/msm_drv.h
>> > > > > > > index 05d33a7..d4cbde2 100644
>> > > > > > > --- a/drivers/gpu/drm/msm/msm_drv.h
>> > > > > > > +++ b/drivers/gpu/drm/msm/msm_drv.h
>> > > > > > > @@ -77,12 +77,6 @@ enum msm_mdp_plane_property {
>> > > > > > >  	PLANE_PROP_MAX_NUM
>> > > > > > >  };
>> > > > > > >
>> > > > > > > -struct msm_vblank_ctrl {
>> > > > > > > -	struct work_struct work;
>> > > > > > > -	struct list_head event_list;
>> > > > > > > -	spinlock_t lock;
>> > > > > > > -};
>> > > > > > > -
>> > > > > > >  #define MSM_GPU_MAX_RINGS 4
>> > > > > > >  #define MAX_H_TILES_PER_DISPLAY 2
>> > > > > > >
>> > > > > > > @@ -225,7 +219,6 @@ struct msm_drm_private {
>> > > > > > >  	struct notifier_block vmap_notifier;
>> > > > > > >  	struct shrinker shrinker;
>> > > > > > >
>> > > > > > > -	struct msm_vblank_ctrl vblank_ctrl;
>> > > > > > >  	struct drm_atomic_state *pm_state;
>> > > > > > >  };
>> > > > > > >
>> > > > > > > --
>> > > > > > > The Qualcomm Innovation Center, Inc. is a member of the Code
>> > Aurora
>> > > > > > Forum,
>> > > > > > > a Linux Foundation Collaborative Project
>> > > > > > >
>> > > > > > > _______________________________________________
>> > > > > > > Freedreno mailing list
>> > > > > > > Freedreno@lists.freedesktop.org
>> > > > > > > https://lists.freedesktop.org/mailman/listinfo/freedreno
>> > > > >
>> > > > > --
>> > > > > Jeykumar S
>> > >
>> > > --
>> > > Jeykumar S
>> 
>> --
>> Jeykumar S

-- 
Jeykumar S
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