linux-kernel - Re: [PATCH] block: fix possible race on blk_get

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Message-ID: <74bd5f51-c8bd-a1b8-cf85-d9eeb0a82d16@acm.org>
Date:   Tue, 28 Jul 2020 19:41:49 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk
Cc:     ming.lei@...hat.com, hch@...radead.org, jack@...e.cz,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: fix possible race on blk_get_queue()

On 2020-07-28 18:51, Luis Chamberlain wrote:
> diff --git a/block/blk-core.c b/block/blk-core.c
> index d9d632639bd1..febdd8e8d409 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -605,12 +605,18 @@ EXPORT_SYMBOL(blk_alloc_queue);
>   */
>  bool blk_get_queue(struct request_queue *q)
>  {
> -	if (likely(!blk_queue_dying(q))) {
> -		__blk_get_queue(q);
> -		return true;
> +	struct kobject *obj;
> +
> +	obj = __blk_get_queue(q);
> +	if (!obj)
> +		return false;
> +
> +	if (unlikely(blk_queue_dying(q))) {
> +		blk_put_queue(q);
> +		return false;
>  	}
>  
> -	return false;
> +	return true;
>  }

This change is not sufficient to prevent that the QUEUE_FLAG_DYING flag
is set immediately after this function returns. I propose not to modify
this function but instead to add a comment that is the responsibility of
the caller to prevent that such a race condition occurs.

> -static inline void __blk_get_queue(struct request_queue *q)
> +static inline struct kobject * __must_check
> +__blk_get_queue(struct request_queue *q)
>  {
> -	kobject_get(&q->kobj);
> +	return kobject_get_unless_zero(&q->kobj);
>  }

If a function passes a queue pointer to another function that calls
blk_get_queue() then the caller should guarantee that 'q' is valid
during the entire duration of the call. In other words, I'm not sure the
above change is an improvement.

Thanks,

Bart.