[dm-devel] [BUG] Oops when SCSI device under multipath is removed

[Alan Stern]

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