Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency

[Will Deacon <will@kernel.org>]

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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