Messages in this thread | | | Date | Wed, 2 Aug 2023 21:37:31 -0400 | Subject | Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency | From | Waiman Long <> |
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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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