Re: [PATCH] sd: use async_probe cookie to avoid deadlocks

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2017-03-21 at 16:25 +0100, Hannes Reinecke wrote:
> On 03/21/2017 02:05 PM, James Bottomley wrote:
> > On Tue, 2017-03-21 at 13:14 +0100, Hannes Reinecke wrote:
> > > With the current design we're waiting for all async probes to
> > > finish when removing any sd device.
> > > This might lead to a livelock where the 'remove' call is blocking
> > > for any probe calls to finish, and the probe calls are waiting
> > > for
> > > a response, which will never be processes as the thread handling
> > > the responses is waiting for the remove call to finish.
> > > Which is completely pointless as we only _really_ care for the
> > > probe on _this_ device to be completed; any other probing can
> > > happily continue for all we care.
> > > So save the async probing cookie in the structure and only wait
> > > if this specific probe is still active.
> > 
> > How does this preserve ordering?  It looks like you have one cookie 
> > per sdkp ... is there some sort of ordering guarantee I'm not
> > seeing?
> > 
> Do we need one?
> The only thing we care here is that probing for _this_ device has 
> finished.

OK, so currently we guarantee the linear ordering luns for individual
hbas.  We also guarantee no interleaving of sdX letters for individual
hbas.  We don't guarantee the scan order of the hbas themselves. 
 Preserve those guarantees and I'm happy with the patch.  If you can't
preserve them I think we need further discussion.

James