Re: [PATCH v2 1/4] drm: Mark up accesses of vblank->enabled outside of its spinlock

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Mar 17, 2017 at 08:20:27PM +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.
> 
> v2: vblank->enable is guarded by dev->vbl_lock not
> dev->vblank_time_lock, update the READ_ONCE accordingly.

The locking is indeed very confusing, and I don't know if it even makes
sense anymore. But this looks at least as sane as anything else here :)

For the series:
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> 
> Do not add a READ_ONCE(vblank->enabled) inside the interrupt handler to
> avoid missing an interrupt whilst racing with enable_vblank()
> 
> 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 | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 53a526c7b24d..c47e07c89136 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -325,6 +325,8 @@ static void vblank_disable_and_save(struct drm_device *dev, unsigned int pipe)
>  	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>  	unsigned long irqflags;
>  
> +	assert_spin_locked(&dev->vbl_lock);
> +
>  	/* Prevent vblank irq processing while disabling vblank irqs,
>  	 * so no updates of timestamps or count can happen after we've
>  	 * disabled. Needed to prevent races in case of delayed irq's.
> @@ -336,10 +338,8 @@ 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
> @@ -384,7 +384,7 @@ 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);
> @@ -1105,11 +1105,16 @@ 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);
>  		}
>  	}
>  
> @@ -1517,7 +1522,7 @@ 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 +1649,7 @@ 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));
>  	}
>  
> -- 
> 2.11.0

-- 
Ville Syrjälä
Intel OTC
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