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

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 09/19] drm: Fix race between drm_vblank_off() and drm_queue_vblank_event()
Date: Wed, 6 Aug 2014 16:33:51 +0300	[thread overview]
Message-ID: <20140806133351.GU4193@intel.com> (raw)
In-Reply-To: <20140806132301.GK8727@phenom.ffwll.local>

On Wed, Aug 06, 2014 at 03:23:01PM +0200, Daniel Vetter wrote:
> On Wed, Aug 06, 2014 at 02:49:52PM +0300, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@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@intel.com>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> I guess the window is too small here to make this reproducible in a test?

I must admit that I didn't even try. I supposed I could try it now.

> 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
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

-- 
Ville Syrjälä
Intel OTC