linux-kernel - Re: mm: GPF in bdi

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Message-ID: <20170302114453.GX29622@ZenIV.linux.org.uk>
Date:   Thu, 2 Mar 2017 11:44:53 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jan Kara <jack@...e.cz>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: mm: GPF in bdi_put

On Wed, Mar 01, 2017 at 03:29:09PM +0100, Jan Kara wrote:

> The problem is writeback code (from flusher work or through sync(2) -
> generally inode_to_bdi() users) can be looking at bdev inode independently
> from it being open. So if they start looking while the bdev is open but the
> dereference happens after it is closed and device removed, we oops. We have
> seen oopses due to this for quite a while. And all the stuff that is done
> in __blkdev_put() is not enough to prevent writeback code from having a
> look whether there is not something to write.

Um.  What's to prevent the queue/device/module itself from disappearing
from under you?  IOW, what are you doing that is safe to do in face of
driver going rmmoded?