Re: [PATCH 01/12] usbnet: introduce usbnet 3 command helpers

[Oliver Neukum <oneukum@suse.de>]

On Wednesday 10 October 2012 17:48:54 Ming Lei wrote:
> On Wed, Oct 10, 2012 at 4:39 PM, Oliver Neukum <oneukum@suse.de> wrote:
> > On Wednesday 10 October 2012 16:17:25 Ming Lei wrote:
> >> On Wed, Oct 10, 2012 at 1:51 PM, Oliver Neukum <oneukum@suse.de> wrote:
> >
> >> > We need to use GFP_NOIO in situations the helper cannot know about.
> >> > Please add a gfp_t parameter. Then the caller will solve that.
> >>
> >> Considered that most of drivers call the helpers in different context, I think
> >> it is better to switch the gpf_t flag runtime inside helpers, like below:
> >>
> >>            if (dev->power.runtime_status == RPM_RESUMING)
> >>                    gfp = GFP_NOIO;
> >>           else
> >>                    gfp = GFP_KERNEL;
> >
> > You are admirably persistent ;-)
> 
> I am only trying to solve the problem more generally, :-)

The most generic solution is passing the parameter.

> > If you extended the check to RPM_SUSPENDING it might work,
> > but still the problem with error handling exists.
> 
> Could you describe the error handling case in a bit detail so
> that callers of these helpers can know when GFP_KERNEL
> is to be passed and when GFP_NOIO is taken if the gfp
> patamerer has to be added?

A reset always applies to the whole device. Resets are used in error
handling of block devices (storage and uas). If you reset a device,
pre_reset() and post_reset() of all interfaces need to be called. So they
are part of the SCSI error handler. SCSI error handlers can allocate memory
only with GFP_NOIO (or GFP_ATOMIC) because any IO for paging
can cause the SCSI layer to wait for the error handling to finish. The error
handling can only finish when pre/post_reset() have finished. Catch-22

So any control messages in block error handling need to use GFP_NOIO.
If you look at the control message helpers in usbcore you will find a lot
of GFP_NOIO. That is the reason.

	Regards
		Oliver