Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path

[Waiman Long]

On 12/12/22 17:24, Tejun Heo wrote:
> On Sun, Dec 11, 2022 at 05:20:58PM -0500, Waiman Long wrote:
> > As noted by Michal, the blkg_iostat_set's in the lockless list
> > hold reference to blkg's to protect against their removal. Those
> > blkg's hold reference to blkcg. When a cgroup is being destroyed,
> > cgroup_rstat_flush() is only called at css_release_work_fn() which is
> > called when the blkcg reference count reaches 0. This circular dependency
> > will prevent blkcg from being freed until some other events cause
> > cgroup_rstat_flush() to be called to flush out the pending blkcg stats.
> >
> > To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
> > function to flush stats for a given css and cpu and call it at the blkgs
> > destruction path, blkcg_destroy_blkgs(), whenever there are still some
> > pending stats to be flushed. This will ensure that blkcg reference
> > count can reach 0 ASAP.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
>
> But a nit below
>
> > +	/*
> > +	 * Flush all the non-empty percpu lockless lists.
> > +	 */
> Can you please explain the deadlock that's being avoided in the above
> comment? ie. it should say why this flush is necessary.

Sure. I will expand the comment to elaborate a bit more.

Cheers,
Longman

> > +	for_each_possible_cpu(cpu) {
> > +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
> > +
> > +		if (!llist_empty(lhead))
> > +			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
> > +	}