diff mbox

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

Message ID 20170316234749.1297-1-chris@chris-wilson.co.uk (mailing list archive)
State New, archived
Headers show

Commit Message

Chris Wilson March 16, 2017, 11:47 p.m. UTC 
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@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_irq.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

Comments

Ville Syrjala March 17, 2017, 9:47 a.m. UTC | #1 
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 <chris@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter@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
diff mbox

Patch

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