Messages in this thread | | | Date | Fri, 28 Jul 2023 16:06:14 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency |
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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?
> + 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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