linux-kernel - Re: warning because of race between userspace and add

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Date:	Wed, 06 Jul 2011 16:56:57 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Roland Dreier <roland@...nel.org>
CC:	open list <linux-kernel@...r.kernel.org>
Subject: Re: warning because of race between userspace and add_disk()?

On 2011-07-06 02:14, Roland Dreier wrote:
> Hi Jens!
> 
> I'm seeing the warning below (with 3.0-rc5) when I hot-add a disk.
> The system has a JBOD with 4 SAS paths, so we get 4 /dev/sdXX for the
> disk (I don't think this is strictly required to trigger this, but I
> think it makes the race bigger).
> 
> What I _think_ is happening is that add_disk() exposes the /dev/sdXX
> device to userspace before it does disk_add_events(), and the
> multipath daemon immediately tries to open the device.  This means
> that blkdev_get() (called from blkdev_open()) races with
> disk_add_events() and so the first call to disk_block_events() returns
> immediately because of
> 
> 	struct disk_events *ev = disk->ev;
> 	if (!ev)
> 		return;
> 
> but then on the way out, disk_unblock_events() finds that disk->ev has
> been set and so hits the warning.
> 
> I guess the fix would be to be more careful about the order we expose
> things when registering disks?

It certainly looks like it, we need to order disk_add_events() before
register_disk() or handle this more gracefully. I'll cook up a patch.

-- 
Jens Axboe

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