Date:   Fri, 4 Aug 2023 12:41:42 -0400
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v3] perf/arm-dmc620: Fix
 dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency

On 8/4/23 12:29, Will Deacon wrote:
> 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@....com>
>>>>> Signed-off-by: Waiman Long <longman@...hat.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.

According to the current code, a refcount of 0 will cause the caller to 
skip the entry and eventually create a new irq itself. That may cause 
the creation of 2 dmc620_pmu_irq structures with the same irq number. 
Will that be a problem? The reason why I see a problem with a refcount 
of 0 because it can now signal both the creation of the new irq or the 
retirement of an old irq that is to be teared down.

Cheers,
Longman

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