On Wed, Jul 21, 2021 at 03:24:13PM +0000, Guido Kiener wrote: > > -----Original Message----- > > From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > > Sent: Wednesday, July 21, 2021 4:22 PM > > To: dave penkler <dpenkler@xxxxxxxxx> > > Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>; qiang.zhang@xxxxxxxxxxxxx; Dmitry > > Vyukov <dvyukov@xxxxxxxxxx>; paulmck@xxxxxxxxxx; Kiener Guido 14DS1 > > <Guido.Kiener@xxxxxxxxxxxxxxxxx>; USB <linux-usb@xxxxxxxxxxxxxxx> > > Subject: *EXT* Re: [PATCH] USB: usbtmc: Fix RCU stall warning > > > > On Wed, Jul 21, 2021 at 11:44:23AM +0200, dave penkler wrote: > > > Sorry, the issue this patch is trying to fix occurs because the > > > current usbtmc driver resubmits the URB when it gets an EPROTO return. > > > The dummy usb host controller driver used in the syzbot tests keeps > > > returning the resubmitted URB with EPROTO causing a loop that starves > > > RCU. With an actual HCI driver it either recovers or returns an EPIPE. > > > > Are you sure about that? Have you ever observed a usbtmc device recovering and > > continuing to operate after an EPROTO error? > > I can't speak for Dave and his investigations. However as you remember I did tests with > EPROTO errors, see thread: https://marc.info/?l=linux-usb&m=162163776930423&w=2 > In the thread you can see the recovering. Ah yes, now I remember. That message doesn't show the _device_ recovering and continuing to operate, though. It shows the _system_ recovering and realizing that the device has been disconnected. What I was asking about was whether you knew of a case where there was an EPROTO error but afterward the usbtmc device continued to work -- no disconnection. Assuming such cases are vanishingly rare, there's no harm in having the driver give up whenever it encounters EPROTO. > Since no user blamed the usbtmc driver for system locks up to now, it's worth to think about > whether the problem is caused by the dummy_hcd driver. Both drivers contributed to the lockup. The question is: Which driver was doing the wrong thing? (Or which was _more_ wrong?) I believe the usbtmc driver was. > I still have no time for further investigations and would agree to use the simple patch > to get rid of the topic for the usbtmc driver. Then the syzbot will maybe find another usb driver. Agreed. So Greg should go ahead and apply the $SUBJECT patch. Alan Stern
