[PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency
Will Deacon
will at kernel.org
Fri Jul 28 08:06:14 PDT 2023
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 at arm.com>
> Signed-off-by: Waiman Long <longman at 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?
> + 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?
Will
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