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Date:   Tue, 3 Jan 2023 11:30:34 +0100
From:   Jan Kara <jack@suse.cz>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     jack@suse.cz, paolo.valente@linaro.org, axboe@kernel.dk,
        tj@kernel.org, josef@toxicpanda.com, linux-block@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        yukuai3@huawei.com, yi.zhang@huawei.com, yangerkun@huawei.com
Subject: Re: [PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount
 apis
Message-ID: <20230103103034.6wtj7fzyoqdsqdqy@quack3>
References: <20230103084755.1256479-1-yukuai1@huaweicloud.com>
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On Tue 03-01-23 16:47:55, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
> during code review, we found that bfq_pd_free() update 'bfqg->ref'
> without holding the lock, which is problematic:
> 
> 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
> 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
> which can concurrent with 1).
> 
> Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
> be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

The patch looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> Changes from RFC:
>  - refcount_dec_and_test(bfqg->ref) -> refcount_dec_and_test(&bfqg->ref)
> 
>  block/bfq-cgroup.c  | 8 +++-----
>  block/bfq-iosched.h | 2 +-
>  2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 1b2829e99dad..7d9b15f0dbd5 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
>  
>  static void bfqg_get(struct bfq_group *bfqg)
>  {
> -	bfqg->ref++;
> +	refcount_inc(&bfqg->ref);
>  }
>  
>  static void bfqg_put(struct bfq_group *bfqg)
>  {
> -	bfqg->ref--;
> -
> -	if (bfqg->ref == 0)
> +	if (refcount_dec_and_test(&bfqg->ref))
>  		kfree(bfqg);
>  }
>  
> @@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
>  	}
>  
>  	/* see comments in bfq_bic_update_cgroup for why refcounting */
> -	bfqg_get(bfqg);
> +	refcount_set(&bfqg->ref, 1);
>  	return &bfqg->pd;
>  }
>  
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 41aa151ccc22..466e4865ace6 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -928,7 +928,7 @@ struct bfq_group {
>  	char blkg_path[128];
>  
>  	/* reference counter (see comments in bfq_bic_update_cgroup) */
> -	int ref;
> +	refcount_t ref;
>  	/* Is bfq_group still online? */
>  	bool online;
>  
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR