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

From: Waiman Long
Date: Wed Aug 02 2023 - 21:46:26 EST

On 8/2/23 21:37, Waiman Long wrote:

On 7/28/23 11:06, Will Deacon wrote:
On Fri, Jul 21, 2023 at 11:17:28PM -0400, Waiman Long wrote:
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@xxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
  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 */
It looks like this can bubble up to the probe() routine. Does the driver
core handle -EAGAIN coming back from a probe routine?
Right, I should add code to handle this error condition. I think it can be handled in dmc620_pmu_get_irq(). The important thing is to release the mutex, wait a few ms and try again. What do you think?

+        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;
Do you actually need a new flag here, or could we use a refcount of zero
to indicate that the irq descriptor is still being constructed?

A refcount of zero can also mean that an existing irq is about to be removed. Right? So I don't think we can use that for this purpose. Besides, there is a 4-byte hole in the structure anyway for arm64.

Alternatively, I can use a special reference count value, say -1, to signal that the irq is not valid yet. What do you think?