[PATCH v2] perf/arm-dmc620: Reverse locking order in dmc620_pmu_get_irq()

[Waiman Long]

The following circular locking dependency was reported when running
cpus online/offline test on an arm64 system.

[   84.195923] Chain exists of:
                 dmc620_pmu_irqs_lock --> cpu_hotplug_lock --> cpuhp_state-down

[   84.207305]  Possible unsafe locking scenario:

[   84.213212]        CPU0                    CPU1
[   84.217729]        ----                    ----
[   84.222247]   lock(cpuhp_state-down);
[   84.225899]                                lock(cpu_hotplug_lock);
[   84.232068]                                lock(cpuhp_state-down);
[   84.238237]   lock(dmc620_pmu_irqs_lock);
[   84.242236]
                *** DEADLOCK ***

The problematic locking order seems to be

	lock(dmc620_pmu_irqs_lock) --> lock(cpu_hotplug_lock)

This locking order happens when dmc620_pmu_get_irq() is called from
dmc620_pmu_device_probe(). Since dmc620_pmu_irqs_lock is used for
protecting the dmc620_pmu_irqs structure only, we don't actually need
to hold the lock when adding a new instance to the CPU hotplug subsystem.

Fix this possible deadlock scenario by releasing the lock when a new
dmc620_pmu_irq needs to be created and reacquring it again when the
new irq is inserted into dmc620_pmu_irqs.

Suggested-by: Robin Murphy <robin.murphy@xxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 drivers/perf/arm_dmc620_pmu.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c
index 9d0f01c4455a..dbf67c122420 100644
--- a/drivers/perf/arm_dmc620_pmu.c
+++ b/drivers/perf/arm_dmc620_pmu.c
@@ -419,13 +419,16 @@ static irqreturn_t dmc620_pmu_handle_irq(int irq_num, void *data)
 }
 
 static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num)
+	__releases(&dmc620_pmu_irqs_lock)
 {
 	struct dmc620_pmu_irq *irq;
 	int ret;
 
 	list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node)
 		if (irq->irq_num == irq_num && refcount_inc_not_zero(&irq->refcount))
-			return irq;
+			goto out_unlock;
+
+	mutex_unlock(&dmc620_pmu_irqs_lock);
 
 	irq = kzalloc(sizeof(*irq), GFP_KERNEL);
 	if (!irq)
@@ -452,8 +455,12 @@ static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num)
 		goto out_free_irq;
 
 	irq->irq_num = irq_num;
+
+	mutex_lock(&dmc620_pmu_irqs_lock);
 	list_add(&irq->irqs_node, &dmc620_pmu_irqs);
 
+out_unlock:
+	mutex_unlock(&dmc620_pmu_irqs_lock);
 	return irq;
 
 out_free_irq:
@@ -469,7 +476,7 @@ static int dmc620_pmu_get_irq(struct dmc620_pmu *dmc620_pmu, int irq_num)
 
 	mutex_lock(&dmc620_pmu_irqs_lock);
 	irq = __dmc620_pmu_get_irq(irq_num);
-	mutex_unlock(&dmc620_pmu_irqs_lock);
+	/* mutex_unlock() called inside __dmc620_pmu_get_irq() */
 
 	if (IS_ERR(irq))
 		return PTR_ERR(irq);
-- 
2.31.1