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Date:   Fri, 28 Jul 2023 16:06:14 +0100
From:   Will Deacon <will@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v3] perf/arm-dmc620: Fix
 dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency
Message-ID: <20230728150614.GF21718@willie-the-truck>
References: <20230722031729.3913953-1-longman@redhat.com>
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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