[Intel-gfx] [PATCH 09/19] drm: Fix race between drm_vblank_off() and drm_queue_vblank_event()

[Daniel Vetter]

On Wed, Aug 06, 2014 at 02:49:52PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Currently it's possible that the following will happen:
> 1. drm_wait_vblank() calls drm_vblank_get()
> 2. drm_vblank_off() gets called
> 3. drm_wait_vblank() calls drm_queue_vblank_event() which
>    adds the event to the queue event though vblank interrupts
>    are currently disabled (and may not be re-enabled ever again).
> 
> To fix the problem, add another vblank->enabled check into
> drm_queue_vblank_event().
> 
> drm_vblank_off() holds event_lock around the vblank disable,
> so no further locking needs to be added to drm_queue_vblank_event().
> vblank disable from another source is not possible since
> drm_wait_vblank() already holds a vblank reference.
> 
> Reviewed-by: Matt Roper <matthew.d.roper at intel.com>
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

I guess the window is too small here to make this reproducible in a test?
Especially since each attempt will take a few hundred ms ...
-Daniel

> ---
>  drivers/gpu/drm/drm_irq.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 9353609..b2428cb 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -1270,6 +1270,7 @@ static int drm_queue_vblank_event(struct drm_device *dev, int pipe,
>  				  union drm_wait_vblank *vblwait,
>  				  struct drm_file *file_priv)
>  {
> +	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>  	struct drm_pending_vblank_event *e;
>  	struct timeval now;
>  	unsigned long flags;
> @@ -1293,6 +1294,18 @@ static int drm_queue_vblank_event(struct drm_device *dev, int pipe,
>  
>  	spin_lock_irqsave(&dev->event_lock, flags);
>  
> +	/*
> +	 * drm_vblank_off() might have been called after we called
> +	 * drm_vblank_get(). drm_vblank_off() holds event_lock
> +	 * around the vblank disable, so no need for further locking.
> +	 * The reference from drm_vblank_get() protects against
> +	 * vblank disable from another source.
> +	 */
> +	if (!vblank->enabled) {
> +		ret = -EINVAL;
> +		goto err_unlock;
> +	}
> +
>  	if (file_priv->event_space < sizeof e->event) {
>  		ret = -EBUSY;
>  		goto err_unlock;
> -- 
> 1.8.5.5
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch