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

[Waiman Long <longman@redhat.com>]

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@arm.com>
>> Signed-off-by: Waiman Long <longman@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 */
> 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.

Cheers,
Longman


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