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

[Alan Stern]

On Mon, Jun 28, 2021 at 06:38:37AM +0000, Zhang, Qiang wrote:
> 
> 
> ________________________________________
> From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Sent: Monday, 19 April 2021 15:27
> To: syzbot; Greg Kroah-Hartman; guido.kiener@xxxxxxxxxxxxxxxxx; dpenkler@xxxxxxxxx; lee.jones@xxxxxxxxxx; USB list
> Cc: bp@xxxxxxxxx; dwmw@xxxxxxxxxxxx; hpa@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; luto@xxxxxxxxxx; mingo@xxxxxxxxxx; syzkaller-bugs@xxxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; x86@xxxxxxxxxx
> Subject: Re: [syzbot] INFO: rcu detected stall in tx
> 
> [Please note: This e-mail is from an EXTERNAL e-mail address]
> 
> On Mon, Apr 19, 2021 at 9:19 AM syzbot
> <syzbot+e2eae5639e7203360018@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    50987bec Merge tag 'trace-v5.12-rc7' of git://git.kernel.o..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1065c5fcd00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=398c4d0fe6f66e68
> > dashboard link: https://syzkaller.appspot.com/bug?extid=e2eae5639e7203360018
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+e2eae5639e7203360018@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > usbtmc 5-1:0.0: unknown status received: -71
> > usbtmc 3-1:0.0: unknown status received: -71
> > usbtmc 5-1:0.0: unknown status received: -71
> 
> >The log shows an infinite stream of these before the stall, so I
> >assume it's an infinite loop in usbtmc.
> >+usbtmc maintainers
> >
> >[  370.171634][    C0] usbtmc 6-1:0.0: unknown status received: >-71
> >[  370.177799][    C1] usbtmc 3-1:0.0: unknown status received: >-71

> This seems like a long time in the following cycle,  when the callback function usbtmc_interrupt() find unknown status error, it will submit urb again. the urb may be insert  urbp_list.
> due to the dummy_timer() be called in bh-disable. 
> This will result in the RCU reading critical area not exiting for a long time (note: bh_disable/enable, preempt_disable/enable is regarded as the RCU critical reading area ), and prevent rcu_preempt kthread be schedule and running.

> Whether to return directly when we find the urb status is unknown error?

Yes.

> diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> index 74d5a9c5238a..39d44339c03f 100644
> --- a/drivers/usb/class/usbtmc.c
> +++ b/drivers/usb/class/usbtmc.c
> @@ -2335,6 +2335,7 @@ static void usbtmc_interrupt(struct urb *urb)
>                 return;
>         default:
>                 dev_err(dev, "unknown status received: %d\n", status);
> +               return;
>         }
>  exit:
>         rv = usb_submit_urb(urb, GFP_ATOMIC);

This is the right thing to do.  In fact, you should also change the code 
above this.  There's no real need for special handling of the 
-ECONNRESET, -ENOENT, ..., -EPIPE codes, since the driver will do the 
same thing no matter what the code is.

Alan Stern