linux-kernel - Re: [PATCH] drm/i915: Use drm_dev

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Date:   Fri, 05 Apr 2019 08:41:16 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        michal.wajdeczko@...el.com, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Janusz Krzysztofik <janusz.krzysztofik@...el.com>
Subject: Re: [PATCH] drm/i915: Use drm_dev_unplug()

Quoting Janusz Krzysztofik (2019-04-05 08:26:57)
> From: Janusz Krzysztofik <janusz.krzysztofik@...el.com>
> 
> The driver does not currently support unbinding from a device which is
> in use.  Since open file descriptors may still be pointing into kernel
> memory where the device structures used to be, entirely correct kernel
> panics protect the driver from being unbound as we should not be
> unbinding it before those dangling pointers have been made safe.
> 
> According to the documentation found inside drivers/gpu/drm/drm_drv.c,
> drm_dev_unplug() should be used instead of drm_dev_unregister() in
> order to make a device inaccessible to users as soon as it is unpluged.
> Follow that advice to make those possibly dangling pointers safe,
> protected by DRM layer from a user who is otherwise left pointing into
> possibly reused kernel memory after the driver has been unbound from
> the device.
> 
> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@...el.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index 9df65d386d11..66163378c481 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -1596,7 +1596,7 @@ static void i915_driver_unregister(struct drm_i915_private *dev_priv)
>         i915_pmu_unregister(dev_priv);
>  
>         i915_teardown_sysfs(dev_priv);
> -       drm_dev_unregister(&dev_priv->drm);
> +       drm_dev_unplug(&dev_priv->drm);

I think we may have our onion inverted here. We want to stop the users
as the first step, then start removing the entries. (That will also
nicely invert the order from register, which is what we typically
expect).

After calling i915_driver_unregister(); call i915_gem_set_wedged() to
immediately (give or take external fences) cancel inflight operations.
-Chris