On Thu, Mar 16, 2017 at 11:47:48PM +0000, Chris Wilson wrote:
> Order the update to vblank->enabled after the timestamp is primed so
> that a concurrent unlocked reader will only see the vblank->enabled with
> the current timestamp.
>
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
> Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
> ---
> drivers/gpu/drm/drm_irq.c | 30 ++++++++++++++++++------------
> 1 file changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 53a526c7b24d..4cc9352ab6a8 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -336,10 +336,8 @@ static void vblank_disable_and_save(struct drm_device
> *dev, unsigned int pipe)
> * calling the ->disable_vblank() operation in atomic context with the
> * hardware potentially runtime suspended.
> */
> - if (vblank->enabled) {
> + if (cmpxchg_relaxed(&vblank->enabled, true, false))
> __disable_vblank(dev, pipe);
> - vblank->enabled = false;
> - }
>
> /*
> * Always update the count and timestamp to maintain the
> @@ -360,7 +358,7 @@ static void vblank_disable_fn(unsigned long arg)
> unsigned long irqflags;
>
> spin_lock_irqsave(&dev->vbl_lock, irqflags);
> - if (atomic_read(&vblank->refcount) == 0 && vblank->enabled) {
> + if (atomic_read(&vblank->refcount) == 0 && READ_ONCE(vblank->enabled)) {
Hmm. Aren't most of these accesses inside the lock? Looks like you're
marking everything READ/WRITE_ONCE()?
> DRM_DEBUG("disabling vblank on crtc %u\n", pipe);
> vblank_disable_and_save(dev, pipe);
> }
> @@ -384,7 +382,7 @@ void drm_vblank_cleanup(struct drm_device *dev)
> for (pipe = 0; pipe < dev->num_crtcs; pipe++) {
> struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>
> - WARN_ON(vblank->enabled &&
> + WARN_ON(READ_ONCE(vblank->enabled) &&
> drm_core_check_feature(dev, DRIVER_MODESET));
>
> del_timer_sync(&vblank->disable_timer);
> @@ -564,7 +562,7 @@ int drm_irq_uninstall(struct drm_device *dev)
> for (i = 0; i < dev->num_crtcs; i++) {
> struct drm_vblank_crtc *vblank = &dev->vblank[i];
>
> - if (!vblank->enabled)
> + if (!READ_ONCE(vblank->enabled))
> continue;
>
> WARN_ON(drm_core_check_feature(dev, DRIVER_MODESET));
> @@ -1105,11 +1103,16 @@ static int drm_vblank_enable(struct drm_device *dev,
> unsigned int pipe)
> */
> ret = __enable_vblank(dev, pipe);
> DRM_DEBUG("enabling vblank on crtc %u, ret: %d\n", pipe, ret);
> - if (ret)
> + if (ret) {
> atomic_dec(&vblank->refcount);
> - else {
> - vblank->enabled = true;
> + } else {
> drm_update_vblank_count(dev, pipe, 0);
> + /* drm_update_vblank_count() includes a wmb so we just
> + * need to ensure that the compiler emits the write
> + * to mark the vblank as enabled after the call
> + * to drm_update_vblank_count().
> + */
> + WRITE_ONCE(vblank->enabled, true);
Ordering+barrier looks correct to me.
> }
> }
>
> @@ -1148,7 +1151,7 @@ static int drm_vblank_get(struct drm_device *dev,
> unsigned int pipe)
> if (atomic_add_return(1, &vblank->refcount) == 1) {
> ret = drm_vblank_enable(dev, pipe);
> } else {
> - if (!vblank->enabled) {
> + if (!READ_ONCE(vblank->enabled)) {
> atomic_dec(&vblank->refcount);
> ret = -EINVAL;
> }
> @@ -1517,7 +1520,7 @@ static int drm_queue_vblank_event(struct drm_device
> *dev, unsigned int pipe,
> * vblank disable, so no need for further locking. The reference from
> * drm_vblank_get() protects against vblank disable from another source.
> */
> - if (!vblank->enabled) {
> + if (!READ_ONCE(vblank->enabled)) {
> ret = -EINVAL;
> goto err_unlock;
> }
> @@ -1644,7 +1647,7 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
> DRM_WAIT_ON(ret, vblank->queue, 3 * HZ,
> (((drm_vblank_count(dev, pipe) -
> vblwait->request.sequence) <= (1 << 23)) ||
> - !vblank->enabled ||
> + !READ_ONCE(vblank->enabled) ||
> !dev->irq_enabled));
> }
>
> @@ -1714,6 +1717,9 @@ bool drm_handle_vblank(struct drm_device *dev, unsigned
> int pipe)
> if (WARN_ON(pipe >= dev->num_crtcs))
> return false;
>
> + if (!READ_ONCE(vblank->enabled))
> + return false;
This to me looks like it could theoretically cause us to
miss an interrupt.
1. enable_irq()
2. drm_update_vblank_count()
3. irq fires
4. drm_handle_vblank() doesn't do anything
5. enabled=true
> +
> spin_lock_irqsave(&dev->event_lock, irqflags);
>
> /* Need timestamp lock to prevent concurrent execution with
> --
> 2.11.0
--
Ville Syrjälä
Intel OTC
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