[PATCH 1/2] drm: Mark up accesses of vblank->enabled outside of its spinlock

[Chris Wilson]

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 <chris at chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel.vetter at 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)) {
 		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);
 		}
 	}
 
@@ -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;
+
 	spin_lock_irqsave(&dev->event_lock, irqflags);
 
 	/* Need timestamp lock to prevent concurrent execution with
-- 
2.11.0