Re: [PATCH] drm/tegra: Fix crash caused by reference count imbalance

[Daniel Vetter]

On Tue, May 17, 2016 at 05:27:15PM +0100, Jon Hunter wrote:
> Commit d2307dea14a4 ("drm/atomic: use connector references (v3)") added
> reference counting for DRM connectors and this caused a crash when
> exercising system suspend on Tegra114 Dalmore.
> 
> The Tegra DSI driver implements a Tegra specific function,
> tegra_dsi_connector_duplicate_state(), to duplicate the connector state
> and destroys the state using the generic helper function,
> drm_atomic_helper_connector_destroy_state(). Following commit
> d2307dea14a4 ("drm/atomic: use connector references (v3)") there is
> now an imbalance in the connector reference count because the Tegra
> function to duplicate state does not take a reference when duplicating
> the state information. However, the generic helper function to destroy
> the state information assumes a reference has been taken and during
> system suspend, when the connector state is destroyed, this leads to a
> crash because we attempt to put the reference for an object that has
> already been freed.
> 
> Fix this by aligning tegra_dsi_connector_duplicate_state() with commit
> d2307dea14a4 ("drm/atomic: use connector references (v3)"), so that we
> take a reference on a connector if crtc is set.
> 
> By fixing tegra_dsi_connector_duplicate_state() to take a reference,
> although a crash was no longer seen, it was then observed that after
> each system suspend-resume cycle, the reference would be one greater
> than before the suspend-resume cycle. Following commit d2307dea14a4
> ("drm/atomic: use connector references (v3)"), it was found that we
> also need to put the reference when calling the function
> tegra_dsi_connector_reset() before freeing the state. Fix this by
> updating tegra_dsi_connector_reset() to call the function
> __drm_atomic_helper_connector_destroy_state() in order to put the
> reference for the connector.
> 
> Finally, add a warning if allocating memory for the state information
> fails in tegra_dsi_connector_reset().
> 
> Fixes: d2307dea14a4 ("drm/atomic: use connector references (v3)")
> 
> Signed-off-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> ---
>  drivers/gpu/drm/tegra/dsi.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tegra/dsi.c b/drivers/gpu/drm/tegra/dsi.c
> index 44e102799195..68aaa4c33cd8 100644
> --- a/drivers/gpu/drm/tegra/dsi.c
> +++ b/drivers/gpu/drm/tegra/dsi.c
> @@ -745,13 +745,18 @@ static void tegra_dsi_soft_reset(struct tegra_dsi *dsi)
>  
>  static void tegra_dsi_connector_reset(struct drm_connector *connector)
>  {
> -	struct tegra_dsi_state *state =
> -		kzalloc(sizeof(*state), GFP_KERNEL);
> +	struct tegra_dsi_state *state = kzalloc(sizeof(*state), GFP_KERNEL);
>  
> -	if (state) {
> +	if (WARN_ON(!state))
> +		return;
> +
> +	if (connector->state) {
> +		__drm_atomic_helper_connector_destroy_state(connector,
> +							    connector->state);
>  		kfree(connector->state);
> -		__drm_atomic_helper_connector_reset(connector, &state->base);
>  	}
> +
> +	__drm_atomic_helper_connector_reset(connector, &state->base);

Please rebase onto drm-misc or linux-next, I've removed the connector
argument from __drm_atomic_helper_connector_destroy_state(). I'll send the
pull request for that later today to Dave.

>  }
>  
>  static struct drm_connector_state *
> @@ -764,6 +769,9 @@ tegra_dsi_connector_duplicate_state(struct drm_connector *connector)
>  	if (!copy)
>  		return NULL;
>  
> +	if (copy->base.crtc)
> +		drm_connector_reference(connector);
> +

Please use __drm_atomic_helper_connector_duplicate_state instead of
open-coding it.

Cheers, Daniel

>  	return &copy->base;
>  }
>  
> -- 
> 2.1.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch