Re: [PATCH] sd: Fix a disk probing hang

[James Bottomley <jejb@linux.vnet.ibm.com>]

On Tue, 2017-11-07 at 22:42 +0000, Bart Van Assche wrote:
> On Tue, 2017-11-07 at 10:09 -0800, James Bottomley wrote:
> > 
> > but can you investigate the root cause rather than trying this
> > bandaid?
> 
> Hello James,
> 
> Thanks for your reply. I think that the root cause is that SCSI
> scanning activity can continue to submit I/O even after
> scsi_remove_host() has unlocked scan_mutex but that
> scsi_remove_host() removes some of the infrastructure that is
> essential to process SCSI requests.

That's not really a useful answer: how does it submit I/O after the
device goes into DEL?  In theory every I/O submitted after this is
returned with an immediate error.  I could buy the fact that we have
pending I/O submitted before we go into DEL, which would argue for some
sort of quiesce wait, but I don't see how I/O submitted after DEL
causes a hang.

>  Are you OK with
> e.g. moving a significant part of scsi_remove_host() into
> scsi_host_dev_release()?

Well not really without seeing the root cause.  Before scsi_forget_host
()it's all about state and after it's just removing some user visible
host attributes, so I can't see how either matters much.
 scsi_forget_host() must be executed from scsi_remove_host() because
that's how the devices go into the DEL state and how we error the
requests without troubling the device driver, so that can't be moved to
release

James