Re: [PATCH drivers/perf: hisi:] drivers/perf: hisi: fix NULL pointer issue when uninstall hns3 pmu driver

[Robin Murphy <robin.murphy@arm.com>]

On 11/10/2023 9:37 am, Jijie Shao wrote:
> 
> on 2023/10/10 17:32, Yicong Yang wrote:
>> Hi Jijie,
>>
>> On 2023/10/9 18:50, Jijie Shao wrote:
>>> From: Hao Chen <chenhao418@huawei.com>
>>>
>>> When uninstall hns3 pmu driver, it will call 
>>> cpuhp_state_remove_instance()
>>> and then callback function hns3_pmu_offline_cpu() is called, it may 
>>> cause
>>> NULL pointer call trace when other driver is installing or uninstalling
>>> concurrently.
>>>
>> More information about the calltrace you've met and how to reproduce 
>> this?
>> I'm not sure why other drivers are involved.
>>
>>> As John Garry's opinion, cpuhp_state_remove_instance() is used for 
>>> shared
>>> interrupt, and using cpuhp_state_remove_instance_nocalls() is fine 
>>> for PCIe
>>> or HNS3 pmu.
>>>
>> I'm a bit confused here. We need to update the using CPU and migrate 
>> the perf
>> context as well as the interrupt affinity in cpuhp::teardown() 
>> callback, so
>> it make sense to not call this on driver detachment. But I cannot figure
>> out why this is related to the shared interrupt, more details?
>>
> ok，I will send v2 to add more details.

This shouldn't have anything to do with concurrency or shared interrupts 
or anything else. It's simply that we should clearly not attempt to 
migrate a PMU context (via invoking the hotplug callbacks) *after* the 
relevant PMU has already been unregistered, since that's liable to lead 
to some kind of use-after-free, and at best it's just a pointless waste 
of time anyway - if we've got to the point of unbinding the driver (or 
failing to probe at all), there should definitely not be any active 
events or other PMU state that needs updating.

Thanks,
Robin.