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Date:   Thu, 26 Nov 2020 19:22:19 +0100
From:   Jan Kara <jack@suse.cz>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>, Coly Li <colyli@suse.de>,
        Mike Snitzer <snitzer@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jan Kara <jack@suse.cz>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        dm-devel@redhat.com, Jan Kara <jack@suse.com>,
        linux-block@vger.kernel.org, linux-bcache@vger.kernel.org,
        linux-mtd@lists.infradead.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, Chao Yu <yuchao0@huawei.com>
Subject: Re: [PATCH 37/44] block: switch partition lookup to use struct
 block_device
Message-ID: <20201126182219.GC422@quack2.suse.cz>
References: <20201126130422.92945-1-hch@lst.de>
 <20201126130422.92945-38-hch@lst.de>
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On Thu 26-11-20 14:04:15, Christoph Hellwig wrote:
>  struct hd_struct *disk_get_part(struct gendisk *disk, int partno)
>  {
> -	struct hd_struct *part;
> +	struct block_device *part;
>  
>  	rcu_read_lock();
>  	part = __disk_get_part(disk, partno);
> -	if (part)
> -		get_device(part_to_dev(part));
> -	rcu_read_unlock();
> +	if (!part) {
> +		rcu_read_unlock();
> +		return NULL;
> +	}
>  
> -	return part;
> +	get_device(part_to_dev(part->bd_part));
> +	rcu_read_unlock();
> +	return part->bd_part;
>  }

This is not directly related to this particular patch but I'm wondering:
What prevents say del_gendisk() from racing with disk_get_part(), so that
delete_partition() is called just after we fetched 'part' pointer and the
last 'part' kobject ref is dropped before disk_get_part() calls
get_device()? I don't see anything preventing that and so we'd hand out
'part' that is soon to be freed (after RCU grace period expires).

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR