[PATCH v6] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency

[Waiman Long <longman@redhat.com>]

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 following locking order happens when dmc620_pmu_get_irq() calls
cpuhp_state_add_instance_nocalls().

	lock(dmc620_pmu_irqs_lock) --> lock(cpu_hotplug_lock)

On the other hand, the calling sequence

  cpuhp_thread_fun()
    => cpuhp_invoke_callback()
      => dmc620_pmu_cpu_teardown()

leads to the locking sequence

	lock(cpuhp_state-down) => lock(dmc620_pmu_irqs_lock)

Here dmc620_pmu_irqs_lock protects both the dmc620_pmu_irqs and the
pmus_node lists in various dmc620_pmu instances. dmc620_pmu_get_irq()
requires protected access to dmc620_pmu_irqs whereas
dmc620_pmu_cpu_teardown() needs protection to the pmus_node lists.
Break this circular locking dependency by using two separate locks to
protect dmc620_pmu_irqs list and the pmus_node lists respectively.

Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 drivers/perf/arm_dmc620_pmu.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c
index 9d0f01c4455a..30cea6859574 100644
--- a/drivers/perf/arm_dmc620_pmu.c
+++ b/drivers/perf/arm_dmc620_pmu.c
@@ -66,8 +66,13 @@
 #define DMC620_PMU_COUNTERn_OFFSET(n) \
 	(DMC620_PMU_COUNTERS_BASE + 0x28 * (n))
 
-static LIST_HEAD(dmc620_pmu_irqs);
+/*
+ * dmc620_pmu_irqs_lock: protects dmc620_pmu_irqs list
+ * dmc620_pmu_node_lock: protects pmus_node lists in all dmc620_pmu instances
+ */
 static DEFINE_MUTEX(dmc620_pmu_irqs_lock);
+static DEFINE_MUTEX(dmc620_pmu_node_lock);
+static LIST_HEAD(dmc620_pmu_irqs);
 
 struct dmc620_pmu_irq {
 	struct hlist_node node;
@@ -475,9 +480,9 @@ static int dmc620_pmu_get_irq(struct dmc620_pmu *dmc620_pmu, int irq_num)
 		return PTR_ERR(irq);
 
 	dmc620_pmu->irq = irq;
-	mutex_lock(&dmc620_pmu_irqs_lock);
+	mutex_lock(&dmc620_pmu_node_lock);
 	list_add_rcu(&dmc620_pmu->pmus_node, &irq->pmus_node);
-	mutex_unlock(&dmc620_pmu_irqs_lock);
+	mutex_unlock(&dmc620_pmu_node_lock);
 
 	return 0;
 }
@@ -486,9 +491,11 @@ static void dmc620_pmu_put_irq(struct dmc620_pmu *dmc620_pmu)
 {
 	struct dmc620_pmu_irq *irq = dmc620_pmu->irq;
 
-	mutex_lock(&dmc620_pmu_irqs_lock);
+	mutex_lock(&dmc620_pmu_node_lock);
 	list_del_rcu(&dmc620_pmu->pmus_node);
+	mutex_unlock(&dmc620_pmu_node_lock);
 
+	mutex_lock(&dmc620_pmu_irqs_lock);
 	if (!refcount_dec_and_test(&irq->refcount)) {
 		mutex_unlock(&dmc620_pmu_irqs_lock);
 		return;
@@ -638,10 +645,10 @@ static int dmc620_pmu_cpu_teardown(unsigned int cpu,
 		return 0;
 
 	/* We're only reading, but this isn't the place to be involving RCU */
-	mutex_lock(&dmc620_pmu_irqs_lock);
+	mutex_lock(&dmc620_pmu_node_lock);
 	list_for_each_entry(dmc620_pmu, &irq->pmus_node, pmus_node)
 		perf_pmu_migrate_context(&dmc620_pmu->pmu, irq->cpu, target);
-	mutex_unlock(&dmc620_pmu_irqs_lock);
+	mutex_unlock(&dmc620_pmu_node_lock);
 
 	WARN_ON(irq_set_affinity(irq->irq_num, cpumask_of(target)));
 	irq->cpu = target;
-- 
2.31.1