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Message-ID: <20201027113532.nriqqws7gdcu5su6@vireshk-i7>
Date: Tue, 27 Oct 2020 17:05:32 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Rob Clark <robdclark@...il.com>
Cc: Daniel Vetter <daniel@...ll.ch>,
dri-devel <dri-devel@...ts.freedesktop.org>,
Rob Clark <robdclark@...omium.org>,
Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
"open list:DRM DRIVER FOR MSM ADRENO GPU"
<linux-arm-msm@...r.kernel.org>,
"open list:DRM DRIVER FOR MSM ADRENO GPU"
<freedreno@...ts.freedesktop.org>,
open list <linux-kernel@...r.kernel.org>,
"Menon, Nishanth" <nm@...com>
Subject: Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path
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
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