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 at 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);
