Re: [PATCH 1/2] drm: fix order of event_lock wrt. vblank_time_clock

[Imre Deak <imre.deak@intel.com>]

On Thu, 2012-11-01 at 00:03 +0200, Imre Deak wrote:
> drm_vblank_off() requires callers to hold the event_lock, while itself
> locking vbl_time and vblank_time_lock. This defines a dependency chain
> that conflicts with the one in drm_handle_vblank() where we first lock
> vblank_time_lock and then the event_lock. Fix this by reversing the
> locking order in drm_handle_vblank().
>
> This should've triggered a lockdep warning in the exynos driver, the
> rest of the drivers were ok, since they are not using drm_vblank_off(),
> or as in the case of the intel driver were not holding the event_lock
> when calling drm_vblank_off(). This latter issue is addressed in the
> next patch.

I just realized this is better solved by fixing the lock order in the
exynos driver. That way we can take the event_lock close to what it
really locks. Fixing things there will also leave the other drivers
unaffected.

I'll follow up with v2 doing this.

--Imre

> 
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/drm_irq.c |   32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> Tested with i915, the rest of the drivers should be checked with
> lockdep enabled.
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 3a3d0ce..2193d4a 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -1236,17 +1236,21 @@ done:
>  	return ret;
>  }
>  
> +/**
> + * drm_handle_vblank_events - send pending vblank events
> + * @dev: DRM device
> + * @crtc: crtc where the vblank(s) happened
> + *
> + * Must be called with @dev->event_lock held.
> + */
>  static void drm_handle_vblank_events(struct drm_device *dev, int crtc)
>  {
>  	struct drm_pending_vblank_event *e, *t;
>  	struct timeval now;
> -	unsigned long flags;
>  	unsigned int seq;
>  
>  	seq = drm_vblank_count_and_time(dev, crtc, &now);
>  
> -	spin_lock_irqsave(&dev->event_lock, flags);
> -
>  	list_for_each_entry_safe(e, t, &dev->vblank_event_list, base.link) {
>  		if (e->pipe != crtc)
>  			continue;
> @@ -1266,8 +1270,6 @@ static void drm_handle_vblank_events(struct drm_device *dev, int crtc)
>  						 e->event.sequence);
>  	}
>  
> -	spin_unlock_irqrestore(&dev->event_lock, flags);
> -
>  	trace_drm_vblank_event(crtc, seq);
>  }
>  
> @@ -1285,21 +1287,22 @@ bool drm_handle_vblank(struct drm_device *dev, int crtc)
>  	s64 diff_ns;
>  	struct timeval tvblank;
>  	unsigned long irqflags;
> +	bool ret = false;
>  
>  	if (!dev->num_crtcs)
>  		return false;
>  
> +	spin_lock_irqsave(&dev->event_lock, irqflags);
> +
>  	/* Need timestamp lock to prevent concurrent execution with
>  	 * vblank enable/disable, as this would cause inconsistent
>  	 * or corrupted timestamps and vblank counts.
>  	 */
> -	spin_lock_irqsave(&dev->vblank_time_lock, irqflags);
> +	spin_lock(&dev->vblank_time_lock);
>  
>  	/* Vblank irq handling disabled. Nothing to do. */
> -	if (!dev->vblank_enabled[crtc]) {
> -		spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
> -		return false;
> -	}
> +	if (!dev->vblank_enabled[crtc])
> +		goto unlock_ret;
>  
>  	/* Fetch corresponding timestamp for this vblank interval from
>  	 * driver and store it in proper slot of timestamp ringbuffer.
> @@ -1340,7 +1343,12 @@ bool drm_handle_vblank(struct drm_device *dev, int crtc)
>  	DRM_WAKEUP(&dev->vbl_queue[crtc]);
>  	drm_handle_vblank_events(dev, crtc);
>  
> -	spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
> -	return true;
> +	ret = true;
> +
> +unlock_ret:
> +	spin_unlock(&dev->vblank_time_lock);
> +	spin_unlock_irqrestore(&dev->event_lock, irqflags);
> +
> +	return ret;
>  }
>  EXPORT_SYMBOL(drm_handle_vblank);