[Intel-gfx] [PATCH v3 06/11] drm/i915: Handle locking better in i915_sink_crc.

[Daniel Vetter]

On Fri, Nov 10, 2017 at 12:34:58PM +0100, Maarten Lankhorst wrote:
> Lock the bare minimum, instead of the entire world, and
> use interruptible locking because we can.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_debugfs.c | 40 +++++++++++++++++++++++++++++--------
>  1 file changed, 32 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 39883cd915db..7e8f40eb970d 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -2736,39 +2736,63 @@ static int i915_sink_crc(struct seq_file *m, void *data)
>  	struct intel_connector *connector;
>  	struct drm_connector_list_iter conn_iter;
>  	struct intel_dp *intel_dp = NULL;
> +	struct drm_modeset_acquire_ctx ctx;
>  	int ret;
>  	u8 crc[6];
>  
> -	drm_modeset_lock_all(dev);
> +	drm_modeset_acquire_init(&ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE);
> +
>  	drm_connector_list_iter_begin(dev, &conn_iter);

I kinda expect this funny locking nesting to upset lockdep, I think we
have a bunch of places where we nest list_iter and modeset locks
differently.

I think the correct nesting is list_iter entirely within the ww_mutex
stuff, which means you'd need to terminate the loop (and remember the
connector), then after list_iter_end do the ww_mutex dance. Of course
that's all assuming CI shows I'm right (hopefully it does since we no
longer reboot, in the past finding these took a few runs until you had the
right test combination to trigger this stuff).

Even if we don't yet have such a case I'd really prefer that list_iter
isn't nested between the acquire_ctx and modeset ww_mutex lockdep
contexts.
-Daniel

> +
>  	for_each_intel_connector_iter(connector, &conn_iter) {
>  		struct drm_crtc *crtc;
> +		struct drm_connector_state *state;
>  
> -		if (!connector->base.state->best_encoder)
> +		if (connector->base.connector_type != DRM_MODE_CONNECTOR_eDP)
>  			continue;
>  
> -		crtc = connector->base.state->crtc;
> -		if (!crtc->state->active)
> +retry:
> +		ret = drm_modeset_lock(&dev->mode_config.connection_mutex, &ctx);
> +		if (ret)
> +			goto err;
> +
> +		state = connector->base.state;
> +		if (!state->best_encoder)
>  			continue;
>  
> -		if (connector->base.connector_type != DRM_MODE_CONNECTOR_eDP)
> +		crtc = state->crtc;
> +		ret = drm_modeset_lock(&crtc->mutex, &ctx);
> +		if (ret)
> +			goto err;
> +
> +		if (!crtc->state->active)
>  			continue;
>  
> -		intel_dp = enc_to_intel_dp(connector->base.state->best_encoder);
> +		intel_dp = enc_to_intel_dp(state->best_encoder);
>  
>  		ret = intel_dp_sink_crc(intel_dp, crc);
>  		if (ret)
> -			goto out;
> +			goto err;
>  
>  		seq_printf(m, "%02x%02x%02x%02x%02x%02x\n",
>  			   crc[0], crc[1], crc[2],
>  			   crc[3], crc[4], crc[5]);
>  		goto out;
> +
> +err:
> +		if (ret == -EDEADLK) {
> +			ret = drm_modeset_backoff(&ctx);
> +			if (!ret)
> +				goto retry;
> +		}
> +		goto out;
>  	}
>  	ret = -ENODEV;
>  out:
>  	drm_connector_list_iter_end(&conn_iter);
> -	drm_modeset_unlock_all(dev);
> +	drm_modeset_drop_locks(&ctx);
> +	drm_modeset_acquire_fini(&ctx);
> +
>  	return ret;
>  }
>  
> -- 
> 2.15.0
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch