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Date:   Sat, 21 Nov 2020 17:24:11 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Jan Kara <jack@suse.cz>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Tejun Heo <tj@kernel.org>, Josef Bacik <josef@toxicpanda.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Coly Li <colyli@suse.de>, Mike Snitzer <snitzer@redhat.com>,
        dm-devel@redhat.com, Richard Weinberger <richard@nod.at>,
        Jan Kara <jack@suse.com>, linux-block@vger.kernel.org,
        xen-devel@lists.xenproject.org, linux-bcache@vger.kernel.org,
        linux-mtd@lists.infradead.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 14/20] block: remove the nr_sects field in struct
 hd_struct
Message-ID: <20201121162411.GA18475@lst.de>
References: <20201118084800.2339180-1-hch@lst.de> <20201118084800.2339180-15-hch@lst.de> <20201119120525.GW1981@quack2.suse.cz> <20201120090820.GD21715@lst.de> <20201120112121.GB15537@quack2.suse.cz> <20201120153253.GA18990@lst.de> <20201120155956.GB4327@casper.infradead.org> <20201120200548.GA27360@quack2.suse.cz>
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On Fri, Nov 20, 2020 at 09:05:48PM +0100, Jan Kara wrote:
> The code is already switched to it AFAICT (the lock is really only used in
> the two places that write i_size). But the problem is that in theory two
> i_size_write() calls can race in a way that the resulting stored i_size is a
> mix of two stored sizes. Now I have hard time imagining how this could
> happen for a block device and if two reconfigurations of a block device
> could race like that we'd have a large problems anyway...

Now that you mention it, yes - i_size_write needs to be under i_rwsem
or an equivalent lock.  We could look into using i_rwsem also for block
device, but for now the spinlock seems to be doing fine.  Note that
in current mainline we only have such a lock protecting i_size of the
block_device inode, but none for the size in hd_struct.