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SubjectRe: [RFC PATCH] blk-cgroup: prevent rcu_sched detected stalls warnings in blkg_destroy_all()
Hello,

Thanks for the fix. A couple comments below.

On Sat, Nov 21, 2020 at 04:34:20PM +0800, Yu Kuai wrote:
> +#define BLKG_DESTROY_BATH 4096

I think you meant BLKG_DESTROY_BATCH.

> static void blkg_destroy_all(struct request_queue *q)
> {
> struct blkcg_gq *blkg, *n;
> + int count = BLKG_DESTROY_BATH;

But might as well just write 4096 here.

> spin_lock_irq(&q->queue_lock);
> list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
> struct blkcg *blkcg = blkg->blkcg;
>
> + /*
> + * If the list is too long, the loop can took a long time,
> + * thus relese the lock for a while when a batch of blkcg
> + * were destroyed.
> + */
> + if (!(--count)) {
> + count = BLKG_DESTROY_BATH;
> + spin_unlock_irq(&q->queue_lock);
> + cond_resched();
> + spin_lock_irq(&q->queue_lock);

You can't continue iteration after dropping both locks. You'd have to jump
out of loop and start list_for_each_entry_safe() again.

> + }
> spin_lock(&blkcg->lock);
> blkg_destroy(blkg);
> spin_unlock(&blkcg->lock);

Thanks.

--
tejun

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