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

On Tue, Jun 29, 2021 at 11:32:36AM +0800, qiang.zhang@windriver.com wrote:

From: Zqiang <redacted>

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 <redacted>

What commit does this fix?  Does it need to go to stable kernels?

What about the usbtmc maintainers, what do they think about this?

thanks,

greg k-h