linux-kernel - Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp

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Date:	Wed, 17 Dec 2014 10:06:17 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Dmitry Torokhov <dtor@...omium.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Stefan Wahren <stefan.wahren@...e.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Nishanth Menon <nm@...com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp_count

On 17 December 2014 at 04:39, Dmitry Torokhov <dtor@...omium.org> wrote:
> A lot of callers are missing the fact that dev_pm_opp_get_opp_count
> needs to be called under RCU lock. Given that RCU locks can safely be
> nested, instead of providing *_locked() API, let's take RCU lock inside

Hmm, I asked for a *_locked() API because many users of
dev_pm_opp_get_opp_count() are already calling it from rcu read side
critical sections.

Now, there are two questions:
- Can rcu-read side critical sections be nested ?

Yes, this is what the comment over rcu_read_lock() says

 * RCU read-side critical sections may be nested.  Any deferred actions
 * will be deferred until the outermost RCU read-side critical section
 * completes.

- Would it be better to drop these double rcu_read_locks() ? i.e. either
get a *_locked() API or fix the callers of dev_pm_opp_get_opp_count().

@Paul: What do you say ?

> dev_pm_opp_get_opp_count() and leave callers as is.
>
> Signed-off-by: Dmitry Torokhov <dtor@...omium.org>
> ---
>  drivers/base/power/opp.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/base/power/opp.c b/drivers/base/power/opp.c
> index 413c7fe..ee5eca2 100644
> --- a/drivers/base/power/opp.c
> +++ b/drivers/base/power/opp.c
> @@ -216,9 +216,7 @@ EXPORT_SYMBOL_GPL(dev_pm_opp_get_freq);
>   * This function returns the number of available opps if there are any,
>   * else returns 0 if none or the corresponding error value.
>   *
> - * Locking: This function must be called under rcu_read_lock(). This function
> - * internally references two RCU protected structures: device_opp and opp which
> - * are safe as long as we are under a common RCU locked section.
> + * Locking: This function takes rcu_read_lock().
>   */
>  int dev_pm_opp_get_opp_count(struct device *dev)
>  {
> @@ -226,13 +224,14 @@ int dev_pm_opp_get_opp_count(struct device *dev)
>         struct dev_pm_opp *temp_opp;
>         int count = 0;
>
> -       opp_rcu_lockdep_assert();
> +       rcu_read_lock();
>
>         dev_opp = find_device_opp(dev);
>         if (IS_ERR(dev_opp)) {
> -               int r = PTR_ERR(dev_opp);
> -               dev_err(dev, "%s: device OPP not found (%d)\n", __func__, r);
> -               return r;
> +               count = PTR_ERR(dev_opp);
> +               dev_err(dev, "%s: device OPP not found (%d)\n",
> +                       __func__, count);
> +               goto out_unlock;
>         }
>
>         list_for_each_entry_rcu(temp_opp, &dev_opp->opp_list, node) {
> @@ -240,6 +239,8 @@ int dev_pm_opp_get_opp_count(struct device *dev)
>                         count++;
>         }
>
> +out_unlock:
> +       rcu_read_unlock();
>         return count;
>  }

Looked fine otherwise:

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
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