[PATCH 3/3] drm: Protect fb_helper list manipulation with a mutex

[Daniel Vetter]

On Wed, Nov 23, 2016 at 02:04:17PM +0000, Chris Wilson wrote:
> Though we only walk the kernel_fb_helper_list inside a panic (or single
> thread debugging), we still need to protect the list manipulation on
> creating/removing a framebuffer device in order to prevent list
> corruption.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

I guess this explains the moved hunk in patch 1. Still feels misplaced,
but with or without moving that around:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index a19afc7eccde..2ac2f462d37b 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -49,6 +49,7 @@ MODULE_PARM_DESC(fbdev_emulation,
>  		 "Enable legacy fbdev emulation [default=true]");
>  
>  static LIST_HEAD(kernel_fb_helper_list);
> +static DEFINE_MUTEX(kernel_fb_helper_lock);
>  
>  /**
>   * DOC: fbdev helpers
> @@ -855,12 +856,14 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
>  	if (!drm_fbdev_emulation)
>  		return;
>  
> +	mutex_lock(&kernel_fb_helper_lock);
>  	if (!list_empty(&fb_helper->kernel_fb_list)) {
>  		list_del(&fb_helper->kernel_fb_list);
>  		if (list_empty(&kernel_fb_helper_list)) {
>  			unregister_sysrq_key('v', &sysrq_drm_fb_helper_restore_op);
>  		}
>  	}
> +	mutex_unlock(&kernel_fb_helper_lock);
>  
>  	drm_fb_helper_crtc_free(fb_helper);
>  
> @@ -2257,10 +2260,12 @@ int drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel)
>  	dev_info(dev->dev, "fb%d: %s frame buffer device\n",
>  		 info->node, info->fix.id);
>  
> +	mutex_lock(&kernel_fb_helper_lock);
>  	if (list_empty(&kernel_fb_helper_list))
>  		register_sysrq_key('v', &sysrq_drm_fb_helper_restore_op);
>  
>  	list_add(&fb_helper->kernel_fb_list, &kernel_fb_helper_list);
> +	mutex_unlock(&kernel_fb_helper_lock);
>  
>  	return 0;
>  }
> -- 
> 2.10.2
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch