Re: [PATCH] PM / devfreq: missing rcu_read_lock() added for find_device_opp()

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting MyungJoo Ham (myungjoo.ham@samsung.com):
> opp_get_notifier() uses find_device_opp(), which requires to
> held rcu_read_lock. In order to keep the notifier-header
> valid, we have added rcu_read_lock().
> 
> Reported-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
> ---
>  drivers/devfreq/devfreq.c |   26 ++++++++++++++++++++------
>  1 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 45e053e..e91cb22 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -1023,11 +1023,18 @@ struct opp *devfreq_recommended_opp(struct device *dev, unsigned long *freq,
>   */
>  int devfreq_register_opp_notifier(struct device *dev, struct devfreq *devfreq)
>  {
> -	struct srcu_notifier_head *nh = opp_get_notifier(dev);
> +	struct srcu_notifier_head *nh;
> +	int ret = 0;
>  
> +	rcu_read_lock();
> +	nh = opp_get_notifier(dev);
>  	if (IS_ERR(nh))
> -		return PTR_ERR(nh);
> -	return srcu_notifier_chain_register(nh, &devfreq->nb);
> +		ret = PTR_ERR(nh);
> +	if (!ret)
> +		ret = srcu_notifier_chain_register(nh, &devfreq->nb);

Hm, but if I'm seeing right, srcu_notifier_chain_register calls
mutex_lock(), which sleeps, so you can't do that under rcu_read_lock().

-serge