Re: [PATCH 1/5] clk: allow reentrant calls into the clk framework

Ulf Hansson
2013-02-28T09:54:38Z

On 28 February 2013 05:49, Mike Turquette <mturquette@linaro.org> wrote:
> Reentrancy into the clock framework from the clk.h api is highly
> desirable. This feature is necessary for clocks that are prepared and
> unprepared via i2c_transfer (which includes many PMICs and discrete
> audio chips) and it is also necessary for performing dynamic voltage &
> frequency scaling via clock rate-change notifiers.
>
> This patch implements reentrancy by adding a global atomic_t which
> tracks the context of the current caller. Context in this case is the
> return value from get_current(). The clk.h api implementations are
> modified to first see if the relevant global lock is already held and if
> so compare the global context (set by whoever is holding the lock)
> against their own context (via a call to get_current()). If the two
> match then this function is a nested call from the one already holding
> the lock and we procede. If the context does not match then procede to
> call mutex_lock and busy-wait for the existing task to complete.
>
> Thus this patch set does not increase concurrency for unrelated calls
> into the clock framework. Instead it simply allows reentrancy by the
> single task which is currently holding the global clock framework lock.
>
> Thanks to Rajagoapl Venkat for the original idea to use get_current()
> and to David Brown for the suggestion to replace my previous rwlock
> scheme with atomic operations during code review at ELC 2013.
>
> Signed-off-by: Mike Turquette <mturquette@linaro.org>
> Cc: Rajagopal Venkat <rajagopal.venkat@linaro.org>
> Cc: David Brown <davidb@codeaurora.org>
> ---
> drivers/clk/clk.c | 254 ++++++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 185 insertions(+), 69 deletions(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index fabbfe1..b7d6a0a 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -19,9 +19,11 @@
> #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 atomic_t context;
>
> static HLIST_HEAD(clk_root_list);
> static HLIST_HEAD(clk_orphan_list);
> @@ -433,27 +435,6 @@ unsigned int __clk_get_prepare_count(struct clk *clk)
> return !clk ? 0 : clk->prepare_count;
> }
>
> -unsigned long __clk_get_rate(struct clk *clk)
> -{
> - unsigned long ret;
> -
> - if (!clk) {
> - ret = 0;
> - goto out;
> - }
> -
> - ret = clk->rate;
> -
> - if (clk->flags & CLK_IS_ROOT)
> - goto out;
> -
> - if (!clk->parent)
> - ret = 0;
> -
> -out:
> - return ret;
> -}
> -
> unsigned long __clk_get_flags(struct clk *clk)
> {
> return !clk ? 0 : clk->flags;
> @@ -524,6 +505,35 @@ struct clk *__clk_lookup(const char *name)
> return NULL;
> }
>
> +/*** locking & reentrancy ***/
> +
> +static void clk_fwk_lock(void)
> +{
> + /* hold the framework-wide lock, context == NULL */
> + mutex_lock(&prepare_lock);
> +
> + /* set context for any reentrant calls */
> + atomic_set(&context, (int) get_current());
> +}
> +
> +static void clk_fwk_unlock(void)
> +{
> + /* clear the context */
> + atomic_set(&context, 0);
> +
> + /* release the framework-wide lock, context == NULL */
> + mutex_unlock(&prepare_lock);
> +}
> +
> +static bool clk_is_reentrant(void)
> +{
> + if (mutex_is_locked(&prepare_lock))
> + if ((void *) atomic_read(&context) == get_current())
> + return true;
> +
> + return false;
> +}
> +
> /*** clk api ***/
>
> void __clk_unprepare(struct clk *clk)
> @@ -558,9 +568,15 @@ void __clk_unprepare(struct clk *clk)
> */
> void clk_unprepare(struct clk *clk)
> {
> - mutex_lock(&prepare_lock);
> + /* re-enter if call is from the same context */
> + if (clk_is_reentrant()) {
> + __clk_unprepare(clk);
> + return;
> + }
> +
> + clk_fwk_lock();
> __clk_unprepare(clk);
> - mutex_unlock(&prepare_lock);
> + clk_fwk_unlock();
> }
> EXPORT_SYMBOL_GPL(clk_unprepare);
>
> @@ -606,10 +622,16 @@ int clk_prepare(struct clk *clk)
> {
> int ret;
>
> - mutex_lock(&prepare_lock);
> - ret = __clk_prepare(clk);
> - mutex_unlock(&prepare_lock);
> + /* re-enter if call is from the same context */
> + if (clk_is_reentrant()) {
> + ret = __clk_prepare(clk);
> + goto out;
> + }
>
> + clk_fwk_lock();
> + ret = __clk_prepare(clk);
> + clk_fwk_unlock();
> +out:
> return ret;
> }

This above code seems fine to me. The slowpath functions using the
prepare lock would be reentrant with this change, which is really
great.

> EXPORT_SYMBOL_GPL(clk_prepare);
> @@ -650,8 +672,27 @@ void clk_disable(struct clk *clk)
> {
> unsigned long flags;
>
> + /* must check both the global spinlock and the global mutex */
> + if (spin_is_locked(&enable_lock) || mutex_is_locked(&prepare_lock)) {
> + if ((void *) atomic_read(&context) == get_current()) {
> + __clk_disable(clk);
> + return;
> + }
> + }
> +
> + /* hold the framework-wide lock, context == NULL */
> spin_lock_irqsave(&enable_lock, flags);
> +
> + /* set context for any reentrant calls */
> + atomic_set(&context, (int) get_current());
> +
> + /* disable the clock(s) */
> __clk_disable(clk);
> +
> + /* clear the context */
> + atomic_set(&context, 0);
> +
> + /* release the framework-wide lock, context == NULL */
> spin_unlock_irqrestore(&enable_lock, flags);
> }
> EXPORT_SYMBOL_GPL(clk_disable);
> @@ -703,10 +744,29 @@ int clk_enable(struct clk *clk)
> unsigned long flags;
> int ret;
>
> + /* this call re-enters if it is from the same context */
> + if (spin_is_locked(&enable_lock) || mutex_is_locked(&prepare_lock)) {
> + if ((void *) atomic_read(&context) == get_current()) {
> + ret = __clk_enable(clk);
> + goto out;
> + }
> + }

I beleive the clk_enable|disable code will be racy. What do you think
about this scenario:

1. Thread 1, calls clk_prepare -> clk is not reentrant -> mutex_lock
-> set_context to thread1.
2. Thread 2, calls clk_enable -> above "if" will mean that get_current
returns thread 1 context and then clk_enable continues ->
spin_lock_irqsave -> set_context to thread 2.
3. Thread 1 continues and triggers a reentancy for clk_prepare -> clk
is not reentrant (since thread 2 has set a new context) -> mutex_lock
and we will hang forever.

Do you think above scenario could happen?

I think the solution would be to invent another "static atomic_t
context;" which is used only for fast path functions
(clk_enable|disable). That should do the trick I think.

> +
> + /* hold the framework-wide lock, context == NULL */
> spin_lock_irqsave(&enable_lock, flags);
> +
> + /* set context for any reentrant calls */
> + atomic_set(&context, (int) get_current());
> +
> + /* enable the clock(s) */
> ret = __clk_enable(clk);
> - spin_unlock_irqrestore(&enable_lock, flags);
>
> + /* clear the context */
> + atomic_set(&context, 0);
> +
> + /* release the framework-wide lock, context == NULL */
> + spin_unlock_irqrestore(&enable_lock, flags);
> +out:
> return ret;
> }
> EXPORT_SYMBOL_GPL(clk_enable);
> @@ -750,10 +810,17 @@ long clk_round_rate(struct clk *clk, unsigned long rate)
> {
> unsigned long ret;
>
> - mutex_lock(&prepare_lock);
> + /* this call re-enters if it is from the same context */
> + if (clk_is_reentrant()) {
> + ret = __clk_round_rate(clk, rate);
> + goto out;
> + }
> +
> + clk_fwk_lock();
> ret = __clk_round_rate(clk, rate);
> - mutex_unlock(&prepare_lock);
> + clk_fwk_unlock();
>
> +out:
> return ret;
> }
> EXPORT_SYMBOL_GPL(clk_round_rate);
> @@ -836,6 +903,30 @@ static void __clk_recalc_rates(struct clk *clk, unsigned long msg)
> __clk_recalc_rates(child, msg);
> }
>
> +unsigned long __clk_get_rate(struct clk *clk)
> +{
> + unsigned long ret;
> +
> + if (!clk) {
> + ret = 0;
> + goto out;
> + }
> +
> + if (clk->flags & CLK_GET_RATE_NOCACHE)
> + __clk_recalc_rates(clk, 0);
> +
> + ret = clk->rate;
> +
> + if (clk->flags & CLK_IS_ROOT)
> + goto out;
> +
> + if (!clk->parent)
> + ret = 0;
> +
> +out:
> + return ret;
> +}
> +
> /**
> * clk_get_rate - return the rate of clk
> * @clk: the clk whose rate is being returned
> @@ -848,14 +939,22 @@ unsigned long clk_get_rate(struct clk *clk)
> {
> unsigned long rate;
>
> - mutex_lock(&prepare_lock);
> + /*
> + * FIXME - any locking here seems heavy weight
> + * can clk->rate be replaced with an atomic_t?
> + * same logic can likely be applied to prepare_count & enable_count
> + */
>
> - if (clk && (clk->flags & CLK_GET_RATE_NOCACHE))
> - __clk_recalc_rates(clk, 0);
> + if (clk_is_reentrant()) {
> + rate = __clk_get_rate(clk);
> + goto out;
> + }
>
> + clk_fwk_lock();
> rate = __clk_get_rate(clk);
> - mutex_unlock(&prepare_lock);
> + clk_fwk_unlock();
>
> +out:
> return rate;
> }
> EXPORT_SYMBOL_GPL(clk_get_rate);
> @@ -1036,6 +1135,39 @@ static void clk_change_rate(struct clk *clk)
> clk_change_rate(child);
> }
>
> +int __clk_set_rate(struct clk *clk, unsigned long rate)
> +{
> + int ret = 0;
> + struct clk *top, *fail_clk;
> +
> + /* bail early if nothing to do */
> + if (rate == clk->rate)
> + return 0;
> +
> + if ((clk->flags & CLK_SET_RATE_GATE) && clk->prepare_count) {
> + return -EBUSY;
> + }
> +
> + /* calculate new rates and get the topmost changed clock */
> + top = clk_calc_new_rates(clk, rate);
> + if (!top)
> + return -EINVAL;
> +
> + /* notify that we are about to change rates */
> + fail_clk = clk_propagate_rate_change(top, PRE_RATE_CHANGE);
> + if (fail_clk) {
> + pr_warn("%s: failed to set %s rate\n", __func__,
> + fail_clk->name);
> + clk_propagate_rate_change(top, ABORT_RATE_CHANGE);
> + return -EBUSY;
> + }
> +
> + /* change the rates */
> + clk_change_rate(top);
> +
> + return ret;
> +}
> +
> /**
> * clk_set_rate - specify a new rate for clk
> * @clk: the clk whose rate is being changed
> @@ -1059,44 +1191,18 @@ static void clk_change_rate(struct clk *clk)
> */
> int clk_set_rate(struct clk *clk, unsigned long rate)
> {
> - struct clk *top, *fail_clk;
> int ret = 0;
>
> - /* prevent racing with updates to the clock topology */
> - mutex_lock(&prepare_lock);
> -
> - /* bail early if nothing to do */
> - if (rate == clk->rate)
> - goto out;
> -
> - if ((clk->flags & CLK_SET_RATE_GATE) && clk->prepare_count) {
> - ret = -EBUSY;
> - goto out;
> - }
> -
> - /* calculate new rates and get the topmost changed clock */
> - top = clk_calc_new_rates(clk, rate);
> - if (!top) {
> - ret = -EINVAL;
> - goto out;
> - }
> -
> - /* notify that we are about to change rates */
> - fail_clk = clk_propagate_rate_change(top, PRE_RATE_CHANGE);
> - if (fail_clk) {
> - pr_warn("%s: failed to set %s rate\n", __func__,
> - fail_clk->name);
> - clk_propagate_rate_change(top, ABORT_RATE_CHANGE);
> - ret = -EBUSY;
> + if (clk_is_reentrant()) {
> + ret = __clk_set_rate(clk, rate);
> goto out;
> }
>
> - /* change the rates */
> - clk_change_rate(top);
> + clk_fwk_lock();
> + ret = __clk_set_rate(clk, rate);
> + clk_fwk_unlock();
>
> out:
> - mutex_unlock(&prepare_lock);
> -
> return ret;
> }
> EXPORT_SYMBOL_GPL(clk_set_rate);
> @@ -1111,10 +1217,16 @@ struct clk *clk_get_parent(struct clk *clk)
> {
> struct clk *parent;
>
> - mutex_lock(&prepare_lock);
> + if (clk_is_reentrant()) {
> + parent = __clk_get_parent(clk);
> + goto out;
> + }
> +
> + clk_fwk_lock();
> parent = __clk_get_parent(clk);
> - mutex_unlock(&prepare_lock);
> + clk_fwk_unlock();
>
> +out:
> return parent;
> }
> EXPORT_SYMBOL_GPL(clk_get_parent);
> @@ -1293,6 +1405,7 @@ out:
> int clk_set_parent(struct clk *clk, struct clk *parent)
> {
> int ret = 0;
> + bool reenter;
>
> if (!clk || !clk->ops)
> return -EINVAL;
> @@ -1300,8 +1413,10 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
> if (!clk->ops->set_parent)
> return -ENOSYS;
>
> - /* prevent racing with updates to the clock topology */
> - mutex_lock(&prepare_lock);
> + reenter = clk_is_reentrant();
> +
> + if (!reenter)
> + clk_fwk_lock();
>
> if (clk->parent == parent)
> goto out;
> @@ -1330,7 +1445,8 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
> __clk_reparent(clk, parent);
>
> out:
> - mutex_unlock(&prepare_lock);
> + if (!reenter)
> + clk_fwk_unlock();
>
> return ret;
> }
> --
> 1.7.10.4
>
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Kind regards
Ulf Hansson