Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path

[Viresh Kumar]

On 25-10-20, 10:39, Rob Clark wrote:
> Nope, I suspect any creation of debugfs files will be problematic.

Yeah, so it only fixed part of the problem.

> (btw, _add_opp_dev_unlocked() looks like it should be called
> _add_opp_dev_locked()?)
> 
> It does look like 'struct opp_table' is already refcnt'd, so I suspect
> you could replace holding opp_table_lock while calling into debugfs
> with holding a reference to the opp_table instead?

It isn't that straight forward unfortunately, we need to make sure the
table doesn't get allocated for the same device twice, so
find+allocate needs to happen within a locked region.

I have taken, not so straight forward, approach to fixing this issue,
lets see if this fixes it or not.

-------------------------8<-------------------------

diff --git a/drivers/opp/core.c b/drivers/opp/core.c
index 4ac4e7ce6b8b..6f4a73a6391f 100644
--- a/drivers/opp/core.c
+++ b/drivers/opp/core.c
@@ -29,6 +29,8 @@
 LIST_HEAD(opp_tables);
 /* Lock to allow exclusive modification to the device and opp lists */
 DEFINE_MUTEX(opp_table_lock);
+/* Flag indicating that opp_tables list is being updated at the moment */
+static bool opp_tables_busy;
 
 static struct opp_device *_find_opp_dev(const struct device *dev,
 					struct opp_table *opp_table)
@@ -1036,8 +1038,8 @@ static void _remove_opp_dev(struct opp_device *opp_dev,
 	kfree(opp_dev);
 }
 
-static struct opp_device *_add_opp_dev_unlocked(const struct device *dev,
-						struct opp_table *opp_table)
+struct opp_device *_add_opp_dev(const struct device *dev,
+				struct opp_table *opp_table)
 {
 	struct opp_device *opp_dev;
 
@@ -1048,7 +1050,9 @@ static struct opp_device *_add_opp_dev_unlocked(const struct device *dev,
 	/* Initialize opp-dev */
 	opp_dev->dev = dev;
 
+	mutex_lock(&opp_table->lock);
 	list_add(&opp_dev->node, &opp_table->dev_list);
+	mutex_unlock(&opp_table->lock);
 
 	/* Create debugfs entries for the opp_table */
 	opp_debug_register(opp_dev, opp_table);
@@ -1056,18 +1060,6 @@ static struct opp_device *_add_opp_dev_unlocked(const struct device *dev,
 	return opp_dev;
 }
 
-struct opp_device *_add_opp_dev(const struct device *dev,
-				struct opp_table *opp_table)
-{
-	struct opp_device *opp_dev;
-
-	mutex_lock(&opp_table->lock);
-	opp_dev = _add_opp_dev_unlocked(dev, opp_table);
-	mutex_unlock(&opp_table->lock);
-
-	return opp_dev;
-}
-
 static struct opp_table *_allocate_opp_table(struct device *dev, int index)
 {
 	struct opp_table *opp_table;
@@ -1121,8 +1113,6 @@ static struct opp_table *_allocate_opp_table(struct device *dev, int index)
 	INIT_LIST_HEAD(&opp_table->opp_list);
 	kref_init(&opp_table->kref);
 
-	/* Secure the device table modification */
-	list_add(&opp_table->node, &opp_tables);
 	return opp_table;
 
 err:
@@ -1135,27 +1125,64 @@ void _get_opp_table_kref(struct opp_table *opp_table)
 	kref_get(&opp_table->kref);
 }
 
+/*
+ * 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_unlocked(dev, 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;
 
 unlock:
 	mutex_unlock(&opp_table_lock);
@@ -1181,6 +1208,10 @@ static void _opp_table_kref_release(struct kref *kref)
 	struct opp_device *opp_dev, *temp;
 	int i;
 
+	/* Drop the lock as soon as we can */
+	list_del(&opp_table->node);
+	mutex_unlock(&opp_table_lock);
+
 	_of_clear_opp_table(opp_table);
 
 	/* Release clk */
@@ -1208,10 +1239,7 @@ static void _opp_table_kref_release(struct kref *kref)
 
 	mutex_destroy(&opp_table->genpd_virt_dev_lock);
 	mutex_destroy(&opp_table->lock);
-	list_del(&opp_table->node);
 	kfree(opp_table);
-
-	mutex_unlock(&opp_table_lock);
 }
 
 void dev_pm_opp_put_opp_table(struct opp_table *opp_table)

-- 
viresh