linux-kernel - Re: [PATCH] block: Check that queue is alive in blk_insert_cloned

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Message-ID: <CAMM=eLfzKuw-2vbERV6D7dAHb1BWopOKZ=dB7vWxcxSCoj6Tkw@mail.gmail.com>
Date:	Mon, 11 Jul 2011 18:40:11 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Roland Dreier <roland@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-scsi@...r.kernel.org,
	Steffen Maier <maier@...ux.vnet.ibm.com>,
	"Manvanthara B. Puttashankar" <manvanth@...ux.vnet.ibm.com>,
	Tarak Reddy <tarak.reddy@...ibm.com>,
	"Seshagiri N. Ippili" <sesh17@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	device-mapper development <dm-devel@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] block: Check that queue is alive in blk_insert_cloned_request()

[cc'ing dm-devel, vivek and tejun]

On Fri, Jul 8, 2011 at 7:04 PM, Roland Dreier <roland@...nel.org> wrote:
> From: Roland Dreier <roland@...estorage.com>
>
> This fixes crashes such as the below that I see when the storage
> underlying a dm-multipath device is hot-removed.  The problem is that
> dm requeues a request to a device whose block queue has already been
> cleaned up, and blk_insert_cloned_request() doesn't check if the queue
> is alive, but rather goes ahead and tries to queue the request.  This
> ends up dereferencing the elevator that was already freed in
> blk_cleanup_queue().

Your patch looks fine to me:
Acked-by: Mike Snitzer <snitzer@...hat.com>

And I looked at various code paths to arrive at the references DM takes.

A reference is taken on the underlying devices' block_device via
drivers/md/dm-table.c:open_dev() with blkdev_get_by_dev().  open_dev()
also does bd_link_disk_holder(), resulting in the mpath device
becoming a holder of the underlying devices. e.g.:
/sys/block/sda/holders/dm-4

But at no point does DM-mpath get a reference to the underlying
devices' request_queue that gets assigned to clone->q (in
drivers/md/dm-mpath.c:map_io).

Seems we should, though AFAIK it won't help with the issue you've
pointed out (because the hotplugged device's driver already called
blk_cleanup_queue and nuked the elevator).

So I'm not sure getting the request_queue reference actually fixes
anything (maybe my imagination is lacking?).  But getting the
request_queue reference is "the right thing" if we're going to be
setting pointers to it.
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