drm/vkms: Fix race-condition between the hrtimer and the atomic commit

Currently, it is possible for the composer to be set as enabled and then
as disabled without a proper call for the vkms_vblank_simulate(). This
is problematic, because the driver would skip one CRC output, causing CRC
tests to fail. Therefore, we need to make sure that, for each time the
composer is set as enabled, a composer job is added to the queue.

In order to provide this guarantee, add a mutex that will lock before
the composer is set as enabled and will unlock only after the composer
job is added to the queue. This way, we can have a guarantee that the
driver won't skip a CRC entry.

This race-condition is affecting the IGT test "writeback-check-output",
making the test fail and also, leaking writeback framebuffers, as the
writeback job is queued, but it is not signaled. This patch avoids both
problems.

[v2]:
    * Create a new mutex and keep the spinlock across the atomic commit in
      order to avoid interrupts that could result in deadlocks.
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230523123207.173976-1-mcanal@igalia.com

drivers/gpu/drm/vkms/vkms_composer.c

@@ -408,10 +408,15 @@ void vkms_set_composer(struct vkms_output *out, bool enabled)
 	if (enabled)
 		drm_crtc_vblank_get(&out->crtc);
 
-	spin_lock_irq(&out->lock);
+	mutex_lock(&out->enabled_lock);
 	old_enabled = out->composer_enabled;
 	out->composer_enabled = enabled;
-	spin_unlock_irq(&out->lock);
+
+	/* the composition wasn't enabled, so unlock the lock to make sure the lock
+	 * will be balanced even if we have a failed commit
+	 */
+	if (!out->composer_enabled)
+		mutex_unlock(&out->enabled_lock);
 
 	if (old_enabled)
 		drm_crtc_vblank_put(&out->crtc);

drivers/gpu/drm/vkms/vkms_crtc.c

@@ -16,7 +16,7 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
 	struct drm_crtc *crtc = &output->crtc;
 	struct vkms_crtc_state *state;
 	u64 ret_overrun;
-	bool ret, fence_cookie;
+	bool ret, fence_cookie, composer_enabled;
 
 	fence_cookie = dma_fence_begin_signalling();
 
@@ -25,15 +25,15 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
 	if (ret_overrun != 1)
 		pr_warn("%s: vblank timer overrun\n", __func__);
 
-	spin_lock(&output->lock);
 	ret = drm_crtc_handle_vblank(crtc);
 	if (!ret)
 		DRM_ERROR("vkms failure on handling vblank");
 
 	state = output->composer_state;
-	spin_unlock(&output->lock);
+	composer_enabled = output->composer_enabled;
+	mutex_unlock(&output->enabled_lock);
 
-	if (state && output->composer_enabled) {
+	if (state && composer_enabled) {
 		u64 frame = drm_crtc_accurate_vblank_count(crtc);
 
 		/* update frame_start only if a queued vkms_composer_worker()
@@ -295,6 +295,7 @@ int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
 
 	spin_lock_init(&vkms_out->lock);
 	spin_lock_init(&vkms_out->composer_lock);
+	mutex_init(&vkms_out->enabled_lock);
 
 	vkms_out->composer_workq = alloc_ordered_workqueue("vkms_composer", 0);
 	if (!vkms_out->composer_workq)

drivers/gpu/drm/vkms/vkms_drv.h

@@ -108,8 +108,10 @@ struct vkms_output {
 	struct workqueue_struct *composer_workq;
 	/* protects concurrent access to composer */
 	spinlock_t lock;
+	/* guarantees that if the composer is enabled, a job will be queued */
+	struct mutex enabled_lock;
 
-	/* protected by @lock */
+	/* protected by @enabled_lock */
 	bool composer_enabled;
 	struct vkms_crtc_state *composer_state;