Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency
From: Waiman Long
Date: Fri Aug 04 2023 - 12:43:37 EST
On 8/4/23 12:29, Will Deacon wrote:
On Wed, Aug 02, 2023 at 09:44:58PM -0400, Waiman Long wrote:
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?
If the device is being removed, we should teardown the irq handler first,
so I don't see why the refcount isn't the right thing.
According to the current code, a refcount of 0 will cause the caller to
skip the entry and eventually create a new irq itself. That may cause
the creation of 2 dmc620_pmu_irq structures with the same irq number.
Will that be a problem? The reason why I see a problem with a refcount
of 0 because it can now signal both the creation of the new irq or the
retirement of an old irq that is to be teared down.
Cheers,
Longman