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

[Waiman Long]

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