Re: Re: Re: Re: Re: [syzbot] INFO: rcu detected stall in tx

[dave penkler]

On Sat, 8 May 2021 at 16:29, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sat, May 08, 2021 at 10:14:41AM +0200, dave penkler wrote:
> > On Thu, 6 May 2021 at 22:31, Guido Kiener
> > <Guido.Kiener@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > -----Original Message-----
> > > > From: Alan Stern
> > > > Sent: Thursday, May 6, 2021 8:32 PM
> > > > To: Kiener Guido 14DS1
> > > >
> > > > On Thu, May 06, 2021 at 05:44:55PM +0000, Guido Kiener wrote:
> > > > > > -----Original Message-----
> > > > > > From: Alan Stern
> > > > > > Sent: Thursday, May 6, 2021 3:49 PM
> > > > > > To: Kiener Guido 14DS1 <Guido.Kiener@xxxxxxxxxxxxxxxxx>
> > > > > > >
> > > > > > > Thanks for your assessment. I agree with the general feeling. I
> > > > > > > counted about hundred specific usb drivers, so wouldn't it be
> > > > > > > better to fix the
> > > > > > problem in some of the host drivers (e.g. urb.c)?
> > > > > > > We could return an error when calling usb_submit_urb() on an erroneous
> > > > pipe.
> > > > > > > I cannot estimate the side effects and we need to check all
> > > > > > > drivers again how they deal with the error situation. Maybe there
> > > > > > > are some special driver
> > > > > > that need a specialized error handling.
> > > > > > > In this case these drivers could reset the (new?) error flag to
> > > > > > > allow calling usb_submit_urb() again without error. This could work, isn't it?
> > > > > >
> > > > > > That is feasible, although it would be an awkward approach.  As you
> > > > > > said, the side effects aren't clear.  But it might work.
> > > > >
> > > > > Otherwise I see only the other approach to change hundred drivers and
> > > > > add the cases EPROTO, EILSEQ and ETIME in each callback handler. The
> > > > > usbtmc driver already respects the EILSEQ and ETIME, and only EPROTO is
> > > > missing.
> > > > > The rest should be more a management task.
> > > > > BTW do you assume it is only a problem for INT pipes or is it also a
> > > > > problem for isochronous and bulk transfers?
> > > >
> > > > All of them.  Control too.
> > > >
> > > > > > Will you be able to test patches?
> > > > >
> > > > > I only can test the USBTMC function in some different PCs. I do not
> > > > > have automated regression tests for USB drivers or Linux kernels.
> > > > > Maybe there is company who could do that.
> > > >
> > > > Well then, if I do find time to write a patch, I'll ask you to try it out with the usbtmc
> > > > driver.
> > >
> > > You mean that you will do a patch in urb.c or a host driver? Or just add a line in usbtmc.c?
> > > Anyhow there is no hurry. On May 20 I will send you a mail if I'm able to
> > > provoke one of these hardware errors EPROTO, EILSQ, or ETIME. Otherwise
> > > it doesn't make sense to test it.
> > >
> > > -Guido
> >
> > EPROTO is a link level issue and needs to be handled by the host driver.
>
> Are you referring to the host controller driver, or to the class device
> driver running on the host?  The host controller driver is responsible
> for creating the -EPROTO error code in the first place.  The class
> device driver is responsible for taking an appropriate action in
> response.
host controller driver
>
> > When the host driver detects a protocol error while processing an URB
> > it completes the URB with EPROTO status and marks the endpoint as
> > halted.
>
> Not true.  It does not mark the endpoint as halted, not unless it
> receives a STALL handshake from the device.  A STALL is not a protocol
> error.
>
> > When the class driver resubmits the URB and the if the host driver
> > finds the endpoint still marked as halted it should return EPIPE
> > status on the resubmitted URB
>
> Irrelevant.
Not at all. The point is that when an application is talking to an
instrument over the usbtmc driver, the underlying host controller and
its driver will detect and silence a babbling endpoint.
Hence no EPROTO loop will ensue in this case and therefore no changes
are needed in usbtmc.
>
> > When the class driver and usbtmc in particular receives an URB with
> > EPIPE status it cleans up and does not resubmit.
> > Can someone from syzbot land please confirm whether usbtmc running on
> > the xhci host driver causes an RCU stall to be detected ?
>
> That is not an easy thing to test, and syzbot is not capable of testing
> it.  You would need a USB device which could deliberately be set to
> create a protocol error; I don't know of any devices like that.
>
> Alan Stern