Messages in this thread | | | Date | Fri, 4 Aug 2023 17:29:55 +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 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@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. > > 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.
Will
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