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

[Guido Kiener]

Hi all,

Dave and I discussed the "self-detected stall on CPU" caused by the usbtmc driver.

What happened?
The callback handler usbtmc_interrupt(struct urb *urb) for the INT pipe receives an erroneous urb with status -EPROTO (-71).
See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/class/usbtmc.c?h=v5.12#n2340
-EPROTO does not abort/shutdown the pipe and the urb is resubmitted to receive the next packet. However the callback handler usbtmc_interrupt is called again with the same erroneous status -EPROTO and this seems to result in an endless loop.
According to https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/driver-api/usb/error-codes.rst?h=v5.12#n177
the error -EPROTO indicates a hardware problem or a bad cable.

Most usb drivers do not react in a specific way on this hardware problems and resubmit the urb. We assume these drivers will run into the same endless loop. Some other driver samples are:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/class/cdc-acm.c?h=v5.12#n379
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/hid/usbhid/usbmouse.c?h=v5.12#n65

Possible solutions:
Hardware defects or bad cables seems to be a common problem for most usb drivers and I assume we do not want to fix this problem in all class specific drivers, but in lower level host drivers, e.g:
1. Using a counter and close the pipe after some detected errors
2. Delay the resubmission of the urb to avoid high cpu usage
3. Do nothing, since it is just a rare problem.

We've never seen this problem in our products and we do not dare to change anything.

- Guido

-----Original Message-----
From: Dmitry
Sent: Monday, April 19, 2021 9:27 AM
Subject: Re: [syzbot] INFO: rcu detected stall in tx

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
[  370.183912][    C0] usbtmc 4-1:0.0: unknown status received: -71
[  370.190076][    C1] usbtmc 5-1:0.0: unknown status received: -71
[  370.196194][    C0] usbtmc 2-1:0.0: unknown status received: -71
[  370.202387][    C1] usbtmc 3-1:0.0: unknown status received: -71
[  370.208460][    C0] usbtmc 6-1:0.0: unknown status received: -71
[  370.214615][    C1] usbtmc 5-1:0.0: unknown status received: -71
[  370.220736][    C0] usbtmc 4-1:0.0: unknown status received: -71
[  370.226902][    C1] usbtmc 3-1:0.0: unknown status received: -71
[  370.233005][    C0] usbtmc 2-1:0.0: unknown status received: -71
[  370.239168][    C1] usbtmc 5-1:0.0: unknown status received: -71
[  370.245271][    C0] usbtmc 6-1:0.0: unknown status received: -71
[  370.251426][    C1] usbtmc 3-1:0.0: unknown status received: -71
[  370.257552][    C0] usbtmc 4-1:0.0: unknown status received: -71
[  370.263715][    C1] usbtmc 5-1:0.0: unknown status received: -71
[  370.269819][    C0] usbtmc 2-1:0.0: unknown status received: -71
[  370.275974][    C1] usbtmc 3-1:0.0: unknown status received: -71
[  370.282100][    C0] usbtmc 6-1:0.0: unknown status received: -71
[  370.288262][    C1] usbtmc 5-1:0.0: unknown status received: -71
[  370.294399][    C0] usbtmc 4-1:0.0: unknown status received: -71