Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

Tejun Heo
2022-06-02T16:59:00Z

Hello,

On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote:
> @@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio)
> }
> bis->cur.ios[rwd]++;
>
> + if (!READ_ONCE(bis->lnode.next)) {
> + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
> +
> + llist_add(&bis->lnode, lhead);
> + percpu_ref_get(&bis->blkg->refcnt);

Hmm... what guarantees that more than one threads race here? llist assumes
that there's a single writer for a given llist_node and the ref count would
be off too, right?

Thanks.

--
tejun