Re: [RFC/PATCH 4/6] OPP: Allow notifiers to call dev_pm_opp_get_{voltage,freq} RCU-free

Krzysztof Kozlowski
2015-06-02T03:31:04Z

On 02.06.2015 10:47, Stephen Boyd wrote:
> We pass the dev_pm_opp structure to OPP notifiers but the users
> of the notifier need to surround calls to dev_pm_opp_get_*() with
> RCU read locks to avoid lockdep warnings. The notifier is already
> called with the dev_opp's srcu lock held, so it should be safe to
> assume the devm_pm_opp structure is already protected inside the
> notifier. Update the lockdep check for this.
>
> Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
> Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
> ---
>
> This is probably wrong, but it's what I had to hack up
> to be able to use the OPP functions from within the notifier
> to figure out the new values of the OPP without having to take
> an RCU read lock.
>
> drivers/base/power/opp.c | 19 +++++++++++--------
> 1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/base/power/opp.c b/drivers/base/power/opp.c
> index b997a7eabcd4..6d75022c6a0e 100644
> --- a/drivers/base/power/opp.c
> +++ b/drivers/base/power/opp.c
> @@ -108,9 +108,10 @@ static LIST_HEAD(dev_opp_list);
> /* Lock to allow exclusive modification to the device and opp lists */
> static DEFINE_MUTEX(dev_opp_list_lock);
>
> -#define opp_rcu_lockdep_assert() \
> +#define opp_rcu_lockdep_assert(s) \
> do { \
> rcu_lockdep_assert(rcu_read_lock_held() || \
> + (s && srcu_read_lock_held(s)) || \
> lockdep_is_held(&dev_opp_list_lock), \
> "Missing rcu_read_lock() or " \
> "dev_opp_list_lock protection"); \
> @@ -169,9 +170,10 @@ unsigned long dev_pm_opp_get_voltage(struct dev_pm_opp *opp)
> struct dev_pm_opp *tmp_opp;
> unsigned long v = 0;
>
> - opp_rcu_lockdep_assert();
> + opp_rcu_lockdep_assert(&opp->dev_opp->srcu_head.srcu);
>
> - tmp_opp = rcu_dereference(opp);
> + tmp_opp = srcu_dereference_check(opp, &opp->dev_opp->srcu_head.srcu,
> + rcu_read_lock_held());

It looks strange. The notifier's SRCU is part of device_opp but here we
want to protect the dev_pm_opp.

Because of this difference I am not sure that it is safe having around
this a srcu_read_lock() from notifiers instead of rcu_read_lock().

Best regards,
Krzysztof