Re: [RFC]extension of the anchor API

[Alan Stern <stern@rowland.harvard.edu>]

On Thu, Apr 15, 2021 at 01:23:21PM +0200, Oliver Neukum wrote:
> Am Mittwoch, den 14.04.2021, 10:56 -0400 schrieb Alan Stern:
> > On Wed, Apr 14, 2021 at 10:12:01AM +0200, Oliver Neukum wrote:
> > > Am Montag, den 12.04.2021, 11:06 -0400 schrieb Alan Stern:
> > > > On Mon, Apr 12, 2021 at 11:58:16AM +0200, Oliver Neukum wrote:
> > > > > That presumes that the URBs will finish in order. I don't think such
> > > > > an assumption can be made.
> > > > 
> > > > I don't understand -- I can't detect any such presumption.
> > > 
> > > OK, this shows that I am bad at explaining.
> > > > As far as I can tell, the only reason for maintaining the URBs in any 
> > > > particular order on the anchor list is so that usb_kill_anchored_urbs 
> > > > and usb_poison_anchored_urbs can kill them in reverse order of 
> > > > submission.  THat's why the current code moves completed URBs to the end 
> > > > of the list.
> > > 
> > > No longer strictly true, as the API has a call to submit everything
> > > on an anchor, but I think it boils down to the same thing.
> > > 
> > > > If you keep a pointer to the most recently submitted URB, killing them 
> > > > easy enough to do.  Start with that URB, then go backward through the 
> > > > list (wrapping to the end when you reach the beginning of the list).
> > > 
> > > Yes, but that supposes that the next on the list has not been
> > > resubmitted _before_ the one after it.
> > > 
> > > If you do not keep the list ordered, but in the initial order,
> > > we can have the situation that A (happens most recently submitted)
> > > is followed by B and C, but C was submitted before B.
> > 
> > I think the only reasonable alternative is to move an URB to the end of 
> > the list when it is submitted, rather than when it completes.  Have you 
> > considered doing it that way?
> 
> No, that did not occur to me. Back to the drawing board.
> Still I have to put it somewhere when I anchor an URB. Head or tail?

Tail, so that the list's order will be the same as the order in which 
URBs were added.

> > The real problem with usb_submit_anchored_urbs is that the core can't 
> > know in what order the caller wants the URBs to be submitted.  If the 
> 
> I think the reasonable assumption is that they need to be submitted
> in the order they were anchored.
> 
> > In the kerneldoc you can explain that if the anchor has not been used 
> > since its URBs were added then the URBs will be submitted in the order 
> > they were added to the anchor, but otherwise they will be submitted in 
> > an unspecified order, which may not be suitable.
> 
> Yes.
> 
> > > > The order in which the URBs complete doesn't matter, because trying to 
> > > > unlink a completed URB won't cause any harm.
> > > 
> > > As long as it stays completed.
> > 
> > Rather, as long as they complete in order of submission.
> > 
> > > >   The only assumption here 
> > > > is that URBs get submitted in the list's order (possibly circularly) -- 
> > > > this should always be true.
> > > 
> > > I am afraid we cannot guarantee that. It might intuitively seem so,
> > > but nothing guarantees that all URBs are going to the same endpoint.
> > 
> > I hadn't thought of that.  Do anchors get used that way anywhere?
> 
> I haven't found an example, but I thought it could not be ruled out.
> So you think that that case should be discouraged in documentation
> and henceforth ignored?

It should work okay.  Really I was just concerned about 
usb_submit_anchored_urbs using the wrong order.  People need to be aware 
that the the order may be wrong.  Put a big warning about it in the 
kerneldoc.

As for reordering the URBs in the list...  I suppose it's unavoidable if 
the URBs can be for different endpoints.  I think it makes more sense to 
do it when the URBs are submitted; that way you know that the list order 
matches the submission order at all times.  But it means you have to be 
careful when submitting all the URBs at once -- especially if a 
completion handler resubmits -- because you want to avoid submitting an 
URB twice.

> So we do agree that we need the following:
> 
> a - submit in the order you
> anchored
> b - kill or poison in the reverse order
> c - unpoison does not really matter but better do it in the submit
> order?

Doesn't matter.  Whatever is easiest.

> Does that mean that the list needs to be kept ordered by sequence
> of submission? I think so.

Yes.

Alan Stern