[PATCH 3/3] drm/vkms: Reduce critical section in vblank_simulate

[Rodrigo Siqueira]

Thanks for this patch! It looks good for me.

Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>

On 07/19, Daniel Vetter wrote:
> We can reduce the critical section in vkms_vblank_simulate under
> output->lock quite a lot:
> 
> - hrtimer_forward_now just needs to be ordered correctly wrt
>   drm_crtc_handle_vblank. We already access the hrtimer timestamp
>   without locks. While auditing that I noticed that we don't correctly
>   annotate the read there, so sprinkle a READ_ONCE to make sure the
>   compiler doesn't do anything foolish.
> 
> - drm_crtc_handle_vblank must stay under the lock to avoid races with
>   drm_crtc_arm_vblank_event.
> 
> - The access to vkms_ouptut->crc_state also must stay under the lock.
> 
> - next problem is making sure the output->state structure doesn't get
>   freed too early. First we rely on a given hrtimer being serialized:
>   If we call drm_crtc_handle_vblank, then we are guaranteed that the
>   previous call to vkms_vblank_simulate has completed. The other side
>   of the coin is that the atomic updates waits for the vblank to
>   happen before it releases the old state. Both taken together means
>   that by the time the atomic update releases the old state, the
>   hrtimer won't access it anymore (it might be accessing the new state
>   at the same time, but that's ok).
> 
> - state is invariant, except the few fields separate protected by
>   state->crc_lock. So no need to hold the lock for that.
> 
> - finally the queue_work. We need to make sure there's no races with
>   the flush_work, i.e. when we call flush_work we need to guarantee
>   that the hrtimer can't requeue the work again. This is guaranteed by
>   the same vblank/hrtimer ordering guarantees like the reasoning above
>   why state won't be freed too early: flush_work on the old state is
>   called after wait_for_flip_done in the atomic commit code.
> 
> Therefore we can also move everything after the output->crc_state out
> of the critical section.
> 
> Motivated by suggestions from Rodrigo.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
> Cc: Haneen Mohammed <hamohammed.sa at gmail.com>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> ---
>  drivers/gpu/drm/vkms/vkms_crtc.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> index 927dafaebc76..74f703b8d22a 100644
> --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> @@ -16,17 +16,18 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
>  	u64 ret_overrun;
>  	bool ret;
>  
> -	spin_lock(&output->lock);
> -
>  	ret_overrun = hrtimer_forward_now(&output->vblank_hrtimer,
>  					  output->period_ns);
>  	WARN_ON(ret_overrun != 1);
>  
> +	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);
> +
>  	if (state && output->composer_enabled) {
>  		u64 frame = drm_crtc_accurate_vblank_count(crtc);
>  
> @@ -48,8 +49,6 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
>  			DRM_DEBUG_DRIVER("Composer worker already queued\n");
>  	}
>  
> -	spin_unlock(&output->lock);
> -
>  	return HRTIMER_RESTART;
>  }
>  
> @@ -85,7 +84,7 @@ bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
>  	struct vkms_output *output = &vkmsdev->output;
>  	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>  
> -	*vblank_time = output->vblank_hrtimer.node.expires;
> +	*vblank_time = READ_ONCE(output->vblank_hrtimer.node.expires);
>  
>  	if (WARN_ON(*vblank_time == vblank->time))
>  		return true;
> -- 
> 2.22.0
> 

-- 
Rodrigo Siqueira
Software Engineer, Advanced Micro Devices (AMD)
https://siqueira.tech
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