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

[Waiman Long]

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.

Cheers,
Longman