[PATCH 2/2] clk: allow reentrant calls into the clk framework

[Ulf Hansson]

On 28 March 2013 21:59, Mike Turquette <mturquette at linaro.org> wrote:
> Reentrancy into the clock framework is necessary for clock operations
> that result in nested calls to the clk api.  A common example is a clock
> that is prepared via an i2c transaction, such as a clock inside of a
> discrete audio chip or a power management IC.  The i2c subsystem itself
> will use the clk api resulting in a deadlock:
>
> clk_prepare(audio_clk)
>         i2c_transfer(..)
>                 clk_prepare(i2c_controller_clk)
>
> The ability to reenter the clock framework prevents this deadlock.
>
> Other use cases exist such as allowing .set_rate callbacks to call
> clk_set_parent to achieve the best rate, or to save power in certain
> configurations.  Yet another example is performing pinctrl operations
> from a clk_ops callback.  Calls into the pinctrl subsystem may call
> clk_{un}prepare on an unrelated clock.  Allowing for nested calls to
> reenter the clock framework enables both of these use cases.
>
> Reentrancy is implemented by two global pointers that track the owner
> currently holding a global lock.  One pointer tracks the owner during
> sleepable, mutex-protected operations and the other one tracks the owner
> during non-interruptible, spinlock-protected operations.
>
> When the clk framework is entered we try to hold the global lock.  If it
> is held we compare the current task against the current owner; a match
> implies a nested call and we reenter.  If the values do not match then
> we block on the lock until it is released.
>
> Signed-off-by: Mike Turquette <mturquette at linaro.org>
> Cc: Rajagopal Venkat <rajagopal.venkat at linaro.org>
> Cc: David Brown <davidb at codeaurora.org>
> Cc: Ulf Hansson <ulf.hansson at linaro.org>
> Tested-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
> ---
> Changes since v5:
>  * fixed up typo in changelog
>
> Changes since v4:
>  * remove uneccesary atomic operations
>  * remove casting bugs
>  * place reentrancy logic into locking helper functions
>  * improve debugging with reference counting and WARNs
>
>  drivers/clk/clk.c |   44 ++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 42 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index 0b5d612..0230c9d 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -19,10 +19,17 @@
>  #include <linux/of.h>
>  #include <linux/device.h>
>  #include <linux/init.h>
> +#include <linux/sched.h>
>
>  static DEFINE_SPINLOCK(enable_lock);
>  static DEFINE_MUTEX(prepare_lock);
>
> +static struct task_struct *prepare_owner;
> +static struct task_struct *enable_owner;
> +
> +static int prepare_refcnt;
> +static int enable_refcnt;
> +
>  static HLIST_HEAD(clk_root_list);
>  static HLIST_HEAD(clk_orphan_list);
>  static LIST_HEAD(clk_notifier_list);
> @@ -30,23 +37,56 @@ static LIST_HEAD(clk_notifier_list);
>  /***           locking             ***/
>  static void clk_prepare_lock(void)
>  {
> -       mutex_lock(&prepare_lock);
> +       if (!mutex_trylock(&prepare_lock)) {
> +               if (prepare_owner == current) {
> +                       prepare_refcnt++;
> +                       return;
> +               }
> +               mutex_lock(&prepare_lock);
> +       }
> +       WARN_ON_ONCE(prepare_owner != NULL);
> +       WARN_ON_ONCE(prepare_refcnt != 0);
> +       prepare_owner = current;
> +       prepare_refcnt = 1;
>  }
>
>  static void clk_prepare_unlock(void)
>  {
> +       WARN_ON_ONCE(prepare_owner != current);
> +       WARN_ON_ONCE(prepare_refcnt == 0);
> +
> +       if (--prepare_refcnt)
> +               return;
> +       prepare_owner = NULL;
>         mutex_unlock(&prepare_lock);
>  }
>
>  static unsigned long clk_enable_lock(void)
>  {
>         unsigned long flags;
> -       spin_lock_irqsave(&enable_lock, flags);
> +
> +       if (!spin_trylock_irqsave(&enable_lock, flags)) {
> +               if (enable_owner == current) {
> +                       enable_refcnt++;
> +                       return flags;
> +               }
> +               spin_lock_irqsave(&enable_lock, flags);
> +       }
> +       WARN_ON_ONCE(enable_owner != NULL);
> +       WARN_ON_ONCE(enable_refcnt != 0);
> +       enable_owner = current;
> +       enable_refcnt = 1;
>         return flags;
>  }
>
>  static void clk_enable_unlock(unsigned long flags)
>  {
> +       WARN_ON_ONCE(enable_owner != current);
> +       WARN_ON_ONCE(enable_refcnt == 0);
> +
> +       if (--enable_refcnt)
> +               return;
> +       enable_owner = NULL;
>         spin_unlock_irqrestore(&enable_lock, flags);
>  }
>
> --
> 1.7.10.4
>

Great piece of work!

Reviewed-by: Ulf Hansson <ulf.hansson at linaro.org>