linux-kernel - Re: [BUG] Oops when SCSI device under multipath is removed

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Message-ID: <Pine.LNX.4.44L0.1108111056540.1958-100000@iolanthe.rowland.org>
Date:	Thu, 11 Aug 2011 10:59:10 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	Jun'ichi Nomura <j-nomura@...jp.nec.com>, <jaxboe@...ionio.com>,
	<roland@...estorage.com>, <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	device-mapper development <dm-devel@...hat.com>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>
Subject: Re: [BUG] Oops when SCSI device under multipath is removed

On Thu, 11 Aug 2011, James Bottomley wrote:

> > If the reason you moved scsi_free_queue into scsi_remove_device
> > is marking the queue dead, how about the following patch?
> > Do you think it's acceptable?
> 
> Well, it's just hiding the problem.  The essential problem is that only
> block has the correctly refcounted knowledge to know the last release of
> the queue reference.  Until that time, the holder of the reference can
> use the queue regardless of whether blk_cleanup_queue() has been called.
> This is the race you complain about since use of the queue involves the
> lock which should be guarded by QUEUE_DEAD checks.
> 
> This is essentially unfixable with function calls.  The only way to fix
> it is to have a callback model for freeing the external lock.

Assuming the queue is associated with a device, the queue could take a
reference to the device, dropping that reference when the queue is
freed.  Then the lock could safely be freed at the same time as the 
device.

Alan Stern

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