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

[Maíra Canal]

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.

Signed-off-by: Maíra Canal <mcanal at igalia.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c |  9 +++++++--
 drivers/gpu/drm/vkms/vkms_crtc.c     | 14 +++++++-------
 drivers/gpu/drm/vkms/vkms_drv.h      |  2 +-
 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 906d3df40cdb..e65104a2203d 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -320,10 +320,15 @@ 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->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->lock);
 
 	if (old_enabled)
 		drm_crtc_vblank_put(&out->crtc);
diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
index 515f6772b866..5a0ee9530748 100644
--- a/drivers/gpu/drm/vkms/vkms_crtc.c
+++ b/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->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()
@@ -241,7 +241,7 @@ static void vkms_crtc_atomic_begin(struct drm_crtc *crtc,
 	/* This lock is held across the atomic commit to block vblank timer
 	 * from scheduling vkms_composer_worker until the composer is updated
 	 */
-	spin_lock_irq(&vkms_output->lock);
+	mutex_lock(&vkms_output->lock);
 }
 
 static void vkms_crtc_atomic_flush(struct drm_crtc *crtc,
@@ -264,7 +264,7 @@ static void vkms_crtc_atomic_flush(struct drm_crtc *crtc,
 
 	vkms_output->composer_state = to_vkms_crtc_state(crtc->state);
 
-	spin_unlock_irq(&vkms_output->lock);
+	mutex_unlock(&vkms_output->lock);
 }
 
 static const struct drm_crtc_helper_funcs vkms_crtc_helper_funcs = {
@@ -290,7 +290,7 @@ int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
 
 	drm_crtc_helper_add(crtc, &vkms_crtc_helper_funcs);
 
-	spin_lock_init(&vkms_out->lock);
+	mutex_init(&vkms_out->lock);
 	spin_lock_init(&vkms_out->composer_lock);
 
 	vkms_out->composer_workq = alloc_ordered_workqueue("vkms_composer", 0);
diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
index 5f1a0a44a78c..883266bb0d4a 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.h
+++ b/drivers/gpu/drm/vkms/vkms_drv.h
@@ -99,7 +99,7 @@ struct vkms_output {
 	/* ordered wq for composer_work */
 	struct workqueue_struct *composer_workq;
 	/* protects concurrent access to composer */
-	spinlock_t lock;
+	struct mutex lock;
 
 	/* protected by @lock */
 	bool composer_enabled;
-- 
2.40.1