Re: [PATCH v3 05/12] smp: Free call_function_data via RCU in smpcfd_dead_cpu

[Chuyi Zhou]

On 2026-03-19 12:19 a.m., Sebastian Andrzej Siewior wrote:
> On 2026-03-18 12:56:31 [+0800], Chuyi Zhou wrote:
>> Use rcu_read_lock() to protect csd in smp_call_function_many_cond() and
>> wait for all read critical sections to exit before releasing percpu csd
>> data. This is preparation for enabling preemption during csd_lock_wait()
>> and can prevent accessing cfd->csd data that has already been freed in
>> smpcfd_dead_cpu().
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
>> ---
>>   kernel/smp.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/smp.c b/kernel/smp.c
>> index 9728ba55944d..32c293d8be0e 100644
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -77,6 +77,7 @@ int smpcfd_dead_cpu(unsigned int cpu)
>>   {
>>   	struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
>>   
>> +	synchronize_rcu();
>>   	free_cpumask_var(cfd->cpumask);
>>   	free_cpumask_var(cfd->cpumask_ipi);
>>   	free_percpu(cfd->csd);
> 
> This seems to make sense. But it could delay CPU shutdown and then the
> stress-cpu-hotplug.sh. And this one helped to find bugs.
> 
> What is expectation of shutting down a CPU? Will it remain off for
> _long_ at which point we care about this memory or is temporary and we
> could skip to free the memory here because we allocate it only once on
> the UP side?
> 


Yes, we can allocate the csd only once during the first UP side. The 
advantage is that it avoids delaying the CPU offline process and keeps 
the code simpler.

The tradeoff is a slight memory waste in scenarios where the CPU remains 
offline for a long time(or never online again). If we cannot tolerate 
the delay caused by synchronize_rcu() during the CPU offline process, we 
can switch to approach you suggested.

Thanks.

> Sebastian