[PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency

[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 before
calling cpuhp_state_add_instance_nocalls() and reacquiring it afterward.
To avoid the possibility of 2 racing dmc620_pmu_get_irq() calls inserting
duplicated dmc620_pmu_irq structures with the same irq number, a dummy
entry is inserted before releasing the lock which will block a competing
thread from inserting another irq structure of the same irq number.

Suggested-by: Robin Murphy <robin.murphy at arm.com>
Signed-off-by: Waiman Long <longman at redhat.com>
---
 drivers/perf/arm_dmc620_pmu.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c
index 9d0f01c4455a..7cafd4dd4522 100644
--- a/drivers/perf/arm_dmc620_pmu.c
+++ b/drivers/perf/arm_dmc620_pmu.c
@@ -76,6 +76,7 @@ struct dmc620_pmu_irq {
 	refcount_t refcount;
 	unsigned int irq_num;
 	unsigned int cpu;
+	unsigned int valid;
 };
 
 struct dmc620_pmu {
@@ -423,9 +424,14 @@ static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num)
 	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))
+	list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node) {
+		if (irq->irq_num != irq_num)
+			continue;
+		if (!irq->valid)
+			return ERR_PTR(-EAGAIN);	/* Try again later */
+		if (refcount_inc_not_zero(&irq->refcount))
 			return irq;
+	}
 
 	irq = kzalloc(sizeof(*irq), GFP_KERNEL);
 	if (!irq)
@@ -447,13 +453,23 @@ static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num)
 	if (ret)
 		goto out_free_irq;
 
-	ret = cpuhp_state_add_instance_nocalls(cpuhp_state_num, &irq->node);
-	if (ret)
-		goto out_free_irq;
-
 	irq->irq_num = irq_num;
 	list_add(&irq->irqs_node, &dmc620_pmu_irqs);
 
+	/*
+	 * Release dmc620_pmu_irqs_lock before calling
+	 * cpuhp_state_add_instance_nocalls() and reacquire it afterward.
+	 */
+	mutex_unlock(&dmc620_pmu_irqs_lock);
+	ret = cpuhp_state_add_instance_nocalls(cpuhp_state_num, &irq->node);
+	mutex_lock(&dmc620_pmu_irqs_lock);
+
+	if (ret) {
+		list_del(&irq->irqs_node);
+		goto out_free_irq;
+	}
+
+	irq->valid = true;
 	return irq;
 
 out_free_irq:
-- 
2.31.1