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SubjectRe: [PATCH 2/3] opp: Allocate the OPP table outside of opp_table_lock
On 02-11-20, 10:51, Viresh Kumar wrote:
> diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> +/*
> + * We need to make sure that the OPP table for a device doesn't get added twice,
> + * if this routine gets called in parallel with the same device pointer.
> + *
> + * The simplest way to enforce that is to perform everything (find existing
> + * table and if not found, create a new one) under the opp_table_lock, so only
> + * one creator gets access to the same. But that expands the critical section
> + * under the lock and may end up causing circular dependencies with frameworks
> + * like debugfs, interconnect or clock framework as they may be direct or
> + * indirect users of OPP core.
> + *
> + * And for that reason we have to go for a bit tricky implementation here, which
> + * uses the opp_tables_busy flag to indicate if another creator is in the middle
> + * of adding an OPP table and others should wait for it to finish.
> + */
> static struct opp_table *_opp_get_opp_table(struct device *dev, int index)
> {
> struct opp_table *opp_table;
>
> - /* Hold our table modification lock here */
> +again:
> mutex_lock(&opp_table_lock);
>
> opp_table = _find_opp_table_unlocked(dev);
> if (!IS_ERR(opp_table))
> goto unlock;
>
> + /*
> + * The opp_tables list or an OPP table's dev_list is getting updated by
> + * another user, wait for it to finish.
> + */
> + if (unlikely(opp_tables_busy)) {
> + mutex_unlock(&opp_table_lock);
> + cpu_relax();
> + goto again;
> + }
> +
> + opp_tables_busy = true;
> opp_table = _managed_opp(dev, index);
> +
> + /* Drop the lock to reduce the size of critical section */
> + mutex_unlock(&opp_table_lock);
> +
> if (opp_table) {
> if (!_add_opp_dev(dev, opp_table)) {
> dev_pm_opp_put_opp_table(opp_table);
> opp_table = ERR_PTR(-ENOMEM);
> }
> - goto unlock;
> +
> + mutex_lock(&opp_table_lock);
> + } else {
> + opp_table = _allocate_opp_table(dev, index);
> +
> + mutex_lock(&opp_table_lock);
> + if (!IS_ERR(opp_table))
> + list_add(&opp_table->node, &opp_tables);
> }
>
> - opp_table = _allocate_opp_table(dev, index);
> + opp_tables_busy = false;

And here is a fix that will be merged with this patch while applying. It is
required as _allocate_opp_table() (which calls _find_table_of_opp_np()) isn't
called with the opp_table_lock anymore.

diff --git a/drivers/opp/of.c b/drivers/opp/of.c
index c718092757d9..6b7f0066942d 100644
--- a/drivers/opp/of.c
+++ b/drivers/opp/of.c
@@ -112,8 +112,6 @@ static struct opp_table *_find_table_of_opp_np(struct device_node *opp_np)
struct opp_table *opp_table;
struct device_node *opp_table_np;

- lockdep_assert_held(&opp_table_lock);
-
opp_table_np = of_get_parent(opp_np);
if (!opp_table_np)
goto err;
@@ -121,12 +119,15 @@ static struct opp_table *_find_table_of_opp_np(struct device_node *opp_np)
/* It is safe to put the node now as all we need now is its address */
of_node_put(opp_table_np);

+ mutex_lock(&opp_table_lock);
list_for_each_entry(opp_table, &opp_tables, node) {
if (opp_table_np == opp_table->np) {
_get_opp_table_kref(opp_table);
+ mutex_unlock(&opp_table_lock);
return opp_table;
}
}
+ mutex_unlock(&opp_table_lock);

err:
return ERR_PTR(-ENODEV);
--
viresh

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