Re: [PATCH] cacheinfo: clear cache_leaves(cpu) in free_cache_attributes()

[Xiongfeng Wang]

Hi Sudeep,

On 2021/7/13 19:33, Sudeep Holla wrote:
> On Tue, Jul 13, 2021 at 11:47:38AM +0800, Xiongfeng Wang wrote:
>> On ARM64, when PPTT(Processor Properties Topology Table) is not
>> implemented in ACPI boot, we will goto 'free_ci' with the following
>> print:
>>   Unable to detect cache hierarchy for CPU 0
>>
> 
> The change itself looks good and I am fine with that. However,...
> 
>> But some other codes may still use 'num_leaves' to iterate through the
> 
> Can you point me exactly where it is used to make sure there are no
> other issues associated with that.
> 
>> 'info_list', such as get_cpu_cacheinfo_id(). If 'info_list' is NULL , it
>> would crash. So clear 'num_leaves' in free_cache_attributes().
>>
> 
> And can you provide the crash dump please ? If we are not hitting any
> issue and you just figured this with code inspection, that is fine. It
> helps to determine if this needs to be backport or just good to have
> clean up.

There is no issue in the mainline kernel. get_cpu_cacheinfo_id() is only called
on x86. I didn't hit any issue using the mainline kernel.

Actually, it's our own code that crashed. My colleague Shaobo(CCed) tried to add
MPAM support on ARM64. His code called get_cpu_cacheinfo_id() and crashed when
PPTT is not implemented. Maybe he should check whether PPTT is implemented
before calling get_cpu_cacheinfo_id(), but we think it is also better to clear
cache_leaves(cpu) in free_cache_attributes().
Sorry for not clearly expressed.

Thanks,
Xiongfeng

> 
>> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
>> ---
>>  drivers/base/cacheinfo.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
>> index bfc0959..dad2962 100644
>> --- a/drivers/base/cacheinfo.c
>> +++ b/drivers/base/cacheinfo.c
>> @@ -297,6 +297,7 @@ static void free_cache_attributes(unsigned int cpu)
>>
>>  	kfree(per_cpu_cacheinfo(cpu));
>>  	per_cpu_cacheinfo(cpu) = NULL;
>> +	cache_leaves(cpu) = 0;
> 
> I initially thought it might get used and crash in cache_shared_cpu_map_remove
> but you are setting it later. So where do you suspect it to be used ? Sorry
> if I am missing something obvious, looking at this code after long time.
>