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Date:   Fri, 27 Nov 2020 16:36:15 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Jan Kara <jack@suse.cz>
Cc:     Christoph Hellwig <hch@lst.de>, 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>,
        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: <20201127153615.GA7524@lst.de>
References: <20201126130422.92945-1-hch@lst.de> <20201126130422.92945-38-hch@lst.de> <20201126182219.GC422@quack2.suse.cz> <20201127094842.GA15984@lst.de> <20201127124537.GC27162@quack2.suse.cz>
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On Fri, Nov 27, 2020 at 01:45:37PM +0100, Jan Kara wrote:
> > At this point the hd_struct is already allocated together with the
> > block_device, and thus only freed after the last block_device reference
> > goes away plus the inode freeing RCU grace period.  So the device model
> > ref to part is indeed gone, but that simply does not matter any more.
> 
> Well, but once device model ref to part is gone, we're going to free the
> bdev inode ref as well. Thus there's nothing which pins the bdev containing
> hd_struct?
> 
> But now as I'm thinking about it you later switch the device model reference
> to just pure inode reference and use igrab() which will reliably return
> NULL if the inode is on it's way to be destroyed so probably we are safe in
> the final state.

igrab always succeeds.  But we should switch to a tryget.