Re: [PATCH-block 3/3] blk-cgroup: Flush stats at blkgs destruction path

Waiman Long
2022-12-09T15:59:36Z

On 12/8/22 18:00, Jens Axboe wrote:
> On 12/8/22 3:01?PM, Waiman Long wrote:
>> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
>> index 793ecff29038..910e633869b0 100644
>> --- a/kernel/cgroup/rstat.c
>> +++ b/kernel/cgroup/rstat.c
>> @@ -281,6 +281,26 @@ void cgroup_rstat_flush_release(void)
>> spin_unlock_irq(&cgroup_rstat_lock);
>> }
>>
>> +/**
>> + * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu
>> + * @css: target css to be flush
>> + * @cpu: the cpu that holds the stats to be flush
>> + *
>> + * A lightweight rstat flush operation for a given css and cpu.
>> + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock
>> + * isn't used.
>> + */
>> +void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu)
>> +{
>> + raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
>> +
>> + raw_spin_lock_irq(cpu_lock);
>> + rcu_read_lock();
>> + css->ss->css_rstat_flush(css, cpu);
>> + rcu_read_unlock();
>> + raw_spin_unlock_irq(cpu_lock);
>> +}
>> +
>> int cgroup_rstat_init(struct cgroup *cgrp)
>> {
>> int cpu;
> As I mentioned last time, raw_spin_lock_irq() will be equivalent to an
> RCU protected section anyway, so you don't need to do both. Just add a
> comment on why rcu_read_lock()/rcu_read_unlock() isn't needed inside the
> raw irq safe lock.

Yes, you are right.  We don't need rcu_read_lock() here. I put it there
to follow the locking pattern in cgroup_rstat_flush_locked(). I will
remove it in the next version.

Cheers,
Longman