On 23/05/23 09:32, Maíra Canal wrote:
> 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@xxxxxxxxxx>
Great catch!
Reviewed-by: Arthur Grillo <arthurgrillo@xxxxxxxxxx>
> ---
> drivers/gpu/drm/vkms/vkms_composer.c | 9 +++++++--
> drivers/gpu/drm/vkms/vkms_crtc.c | 9 +++++----
> drivers/gpu/drm/vkms/vkms_drv.h | 4 +++-
> 3 files changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index 906d3df40cdb..b12188fd6b38 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->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);
> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> index 515f6772b866..3079013c8b32 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->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()
> @@ -292,6 +292,7 @@ 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)
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index 5f1a0a44a78c..dcf4e302fb4d 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -100,8 +100,10 @@ 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;
>
