linux-kernel - Re: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race

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Message-ID: <20160829205439.GG28713@mtj.duckdns.org>
Date:   Mon, 29 Aug 2016 16:54:39 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     Rabin Vincent <rabin@....in>, Jens Axboe <axboe@...com>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race

Hello,

On Mon, Aug 29, 2016 at 10:49:57PM +0200, Vegard Nossum wrote:
> That didn't work at all. I guess bd_acquire() would just do a bdgrab()
> and not touch ->bd_holders, whereas blkdev_get() would increment

Yeah, bdev has two different refs - one for bdev struct itself and the
other for the actual accessors.

> bd_holders and therefore prevent __blkdev_put() from freeing the block
> device? Too confusing...

I think you'll need to do the actual blkdev_get or one of its
variants.

Thanks.

-- 
tejun