[PATCH] drm: Do not call drm_dev_unregister twice on drm_unplug_dev

[Hans de Goede]

On 28-05-17 19:10, Hans de Goede wrote:
> Since commit a39be606f99d ("drm: Do a full device unregister when
> unplugging") drm_unplug_dev has been calling drm_dev_unregister followed
> by a drm_put_dev when open_count reaches 0. This drm_put_dev calls
> drm_dev_unregister again. Since drm_dev_unregister is not protected
> against being called multiple times this leads to havoc.
> 
> This commit fixes this by calling drm_dev_unref instead of drm_put_dev.
> 
> This commit also moves the mutex_lock(&drm_global_mutex) to above the
> drm_dev_unregister call to avoid a race between drm_unplug_dev and
> drm_release still leading to drm_dev_unregister getting called twice.
> 
> Note that calling drm_dev_unregister with the drm_global_mutex locked is
> fine, drm_release already did this (through drm_put_dev).

Ugh I just realized that the whole moving of the mutex thing is not
necessary as I ended up changing the drm_put_dev in drm_release to
drm_dev_unref too. I will send a v2 shortly.

Regards,

Hans



> 
> Fixes: a39be606f99d ("drm: Do a full device unregister when unplugging")
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Marco Diego Aurélio Mesquita <marcodiegomesquita at gmail.com>
> Reported-by: Marco Diego Aurélio Mesquita <marcodiegomesquita at gmail.com>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> Note I don't have any USB display devices at hand for testing atm so
> this patch has only been compile tested.
> ---
>   drivers/gpu/drm/drm_drv.c  | 10 +++++-----
>   drivers/gpu/drm/drm_file.c |  2 +-
>   2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index b5c6bb46a425..9f31d5d629d1 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -357,16 +357,16 @@ EXPORT_SYMBOL(drm_put_dev);
>   
>   void drm_unplug_dev(struct drm_device *dev)
>   {
> +	mutex_lock(&drm_global_mutex);
> +
>   	/* for a USB device */
>   	drm_dev_unregister(dev);
>   
> -	mutex_lock(&drm_global_mutex);
> -
>   	drm_device_set_unplugged(dev);
>   
> -	if (dev->open_count == 0) {
> -		drm_put_dev(dev);
> -	}
> +	if (dev->open_count == 0)
> +		drm_dev_unref(dev);
> +
>   	mutex_unlock(&drm_global_mutex);
>   }
>   EXPORT_SYMBOL(drm_unplug_dev);
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index 3783b659cd38..edba71c8ccc3 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -424,7 +424,7 @@ int drm_release(struct inode *inode, struct file *filp)
>   	if (!--dev->open_count) {
>   		drm_lastclose(dev);
>   		if (drm_device_is_unplugged(dev))
> -			drm_put_dev(dev);
> +			drm_dev_unref(dev);
>   	}
>   	mutex_unlock(&drm_global_mutex);
>   
>