Re: [PATCH] USB: usbtmc: Fix RCU stall warning

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jul 21, 2021 at 11:44:23AM +0200, dave penkler wrote:
> On Wed, 21 Jul 2021 at 09:52, Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Jul 21, 2021 at 09:41:15AM +0200, dave penkler wrote:
> > > On Wed, 21 Jul 2021 at 09:08, Greg KH <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Tue, Jun 29, 2021 at 11:32:36AM +0800, qiang.zhang@windriver.com wrote:
> > > > > From: Zqiang <qiang.zhang@windriver.com>
> > > >
> > > > I need a "full" name here, and in the signed-off-by line please.
> > > >
> > > > >
> > > > > rcu: INFO: rcu_preempt self-detected stall on CPU
> > > > > rcu:    1-...!: (2 ticks this GP) idle=d92/1/0x4000000000000000
> > > > >         softirq=25390/25392 fqs=3
> > > > >         (t=12164 jiffies g=31645 q=43226)
> > > > > rcu: rcu_preempt kthread starved for 12162 jiffies! g31645 f0x0
> > > > >      RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
> > > > > rcu:    Unless rcu_preempt kthread gets sufficient CPU time,
> > > > >         OOM is now expected behavior.
> > > > > rcu: RCU grace-period kthread stack dump:
> > > > > task:rcu_preempt     state:R  running task
> > > > >
> > > > > In the case of system use dummy_hcd as usb controller, when the
> > > > > usbtmc devices is disconnected, in usbtmc_interrupt(), if the urb
> > > > > status is unknown, the urb will be resubmit, the urb may be insert
> > > > > to dum_hcd->urbp_list again, this will cause the dummy_timer() not
> > > > > to exit for a long time, beacause the dummy_timer() be called in
> > > > > softirq and local_bh is disable, this not only causes the RCU reading
> > > > > critical area to consume too much time but also makes the tasks in
> > > > > the current CPU runq not run in time, and that triggered RCU stall.
> > > > >
> > > > > return directly when find the urb status is not zero to fix it.
> > > > >
> > > > > Reported-by: syzbot+e2eae5639e7203360018@syzkaller.appspotmail.com
> > > > > Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> > > >
> > > > What commit does this fix?  Does it need to go to stable kernels?
> > > >
> > > > What about the usbtmc maintainers, what do they think about this?
> > >
> > > This patch makes the babbling endpoint retry/recovery code in the real
> > > world usb host controller drivers redundant and would prevent usbtmc
> > > applications from benefiting from it.
> >
> > I do not understand, is this change ok or not?
> >
> > Why do usbtmc applications need to know if babbling happens or not?
> >
> > confused,
> 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.
> In either case no loop occurs. So for my part as a user and maintainer
> this patch is not ok.

Thanks for the review.

Zqiang, can you fix this patch up based on this please?

thanks,

greg k-h