linux-kernel - Re: [PATCH] blk-cgroup: check blkcg policy is enabled in blkg

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Date:   Mon, 11 Oct 2021 17:23:18 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH] blk-cgroup: check blkcg policy is enabled in
 blkg_create()

Hello.

On Fri, Oct 08, 2021 at 03:27:20PM +0800, Yu Kuai <yukuai3@...wei.com> wrote:
> This is because blkg_alloc() is called from blkg_conf_prep() without
> holding 'q->queue_lock', and elevator is exited before blkg_create():
 
IIUC the problematic interleaving is this one (I've noticed `blkg->pd[i]
= NULL` to thread 2 call trace):

> thread 1                            thread 2
> blkg_conf_prep
>  spin_lock_irq(&q->queue_lock);
>  blkg_lookup_check -> return NULL
>  spin_unlock_irq(&q->queue_lock);
> 
>  blkg_alloc
>   blkcg_policy_enabled -> true
>   pd = ->pd_alloc_fn
>                                    blk_mq_exit_sched
>                                     bfq_exit_queue
>                                      blkcg_deactivate_policy
>                                       spin_lock_irq(&q->queue_lock);
>                                       __clear_bit(pol->plid, q->blkcg_pols);
>
                                        pol->pd_free_fn(blkg->pd[i]);
                                        blkg->pd[i] = NULL;
>
>                                       spin_unlock_irq(&q->queue_lock);
>                                     q->elevator = NULL;
    blkg->pd[i] = pd
>   spin_lock_irq(&q->queue_lock);
>    blkg_create
>     if (blkg->pd[i])
>      ->pd_init_fn -> q->elevator is NULL
>   spin_unlock_irq(&q->queue_lock);

In high-level terms, is this a race between (blk)io controller attribute
write and a device scheduler (elevator) switch?
If so, I'd add it to the commit message.

> Fix the problem by checking that policy is still enabled in
> blkg_create().

Is this sufficient wrt some other q->elevator users later?

> @@ -252,6 +266,9 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
>  		goto err_free_blkg;
>  	}
>  

I'd add a comment here like:

> Re-check policies are still enabled, since the caller blkg_conf_prep()
> temporarily drops q->queue_lock and we can race with
> blk_mq_exit_sched() removing policies.

> +	if (new_blkg)
> +		blkg_check_pd(q, new_blkg);
> +

Thanks,
Michal