On Wed, 21 Jul 2021 at 09:52, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Tue, Jun 29, 2021 at 11:32:36AM +0800, qiang.zhang@xxxxxxxxxxxxx wrote: > > > > From: Zqiang <qiang.zhang@xxxxxxxxxxxxx> > > > > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx> > > > > > > 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. > > greg k-h
