Re: Discussion: soft unbinding

Stefan Richter
2008-05-03T17:22:24Z

Alan Stern wrote:
> When talking about "soft" unbinding, the main question seems to be: How
> soft?
>
> It would be easy, for instance, to change usb-storage so that unbinding
> would wait until the current command was finished. But clearly one
> wants to do more: Give the upper-level SCSI drivers a chance to
> shutdown cleanly and issue their FLUSH CACHE commands, wait for all
> pending commands to complete, and so on.

scsi_remove_host is potentially able to do this, and unless my memory
betrays me, did so in the past.

> It's the "wait for pending commands to complete" part that is hard.
> Some commands have relatively long timeouts.

Is there reason to be less patient during soft unbinding?

If so, the decision which commands can be aborted should IMO be made by
the application layer.

> Error handler operations have no timeouts. Commands submitted through
> sg can have effectively infinite timeouts.

Hmm, I can't comment on these two.

> So how long should we wait?

I presume if a user launches a "remove safely" command, he means it. Or
if he doesn't mean it, he still can hot-unplug before completion of the
shutdown procedures. The only exception is a locked drive door or a
similar ejection mechanism which forces the user to wait for software
coming to terms.

> Should there be a scsi_soft_remove_host() routine that accepts a
> timeout value? It would remove the devices under the host and wait
> until the timeout expires (if necessary) before aborting all pending
> commands. Unlike scsi_remove_host(), it would really abort these
> commands as though they had timed out, instead of simply cancelling
> them. It would guarantee that when it returned, no commands were still
> running on the host and no more commands would be submitted.

It would be an API with more guarantees/ clearer semantics than
scsi_remove_host() and even also...

> This would essentially be a standardized version of the special code
> Stefan has put into the sbp2 and firewire-sbp2 drivers.

...with more guarantees/ clearer semantics than the scsi_remove_device()
API which the SBP-2 drivers happen to use. They use them merely because
this has been found to work more satisfyingly at some point, and they
don't have difficulties to use this API (i.e. look up the logical units
to feed to scsi_remove_device()).

Curious; scsi_mid_low_api.txt says in the context of scsi_remove_host:

When an HBA is being removed it could be as part of an orderly
shutdown associated with the LLD module being unloaded (e.g. with
the "rmmod" command) or in response to a "hot unplug" indicated by
sysfs()'s remove() callback being invoked. In either case, the
sequence is the same [...]

while it says in the context of scsi_remove_device:

In a similar fashion, an LLD may become aware that a SCSI device has
been removed (unplugged) or the connection to it has been
interrupted. [...] An LLD that detects the removal of a SCSI device
can instigate its removal from upper layers with this sequence [...]

AFAIR scsi_remove_host once simply worked just as if the LLD itself
called scsi_remove_device() for each device on that host beforehand.
Eventually there was a change in the SCSI core internal state model
which reduced what scsi_remove_device(), when called internally from
within scsi_remove_host(), was able to do. This is contrary to the text
quoted above. I haven't tested for some time now how the SCSI core
behaves right nowadays.

Back to scsi_soft_remove_host():

Does the SCSI core actually need separate APIs for soft unbinding
(a.k.a. orderly shutdown) and hot removal? We surely have different
requirements in both cases: Give pending commands some time to finish
and send some finalizing commands (e.g. synchronize cache, unlock door)
in the shutdown case, fail all commands and stop any error retries in
the hot unplug case.

But isn't hot unplug just a special case of orderly shutdown ---
basically a case where the transport driver's responsibility is to fail
commands (pending ones and new ones) quickly? In addition, fail them
with failure indicators which tell upper layers that it is no use to
retry them.

Actually, quick failure and suppression of retries in the hot unplug
case is IMO not even as critical as the proper execution of pending and
finalizing commands in the soft unbinding case. The only critical
aspect of hot unplug is that IO terminates eventually, i.e. applications
don't hang.

So, rather than adding a scsi_soft_remove_host API, wouldn't it be
appropriate and possible to make sure that

- scsi_remove_host is able to initiate and perform soft unbinding,

- LLDs return proper failure codes in the hot unplug case, and SCSI
core and upper layers properly interpret them i.e. don't initiate
futile retries.
--
Stefan Richter
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