linux-kernel - Re: [syzbot] [usb?] INFO: rcu detected stall in raw

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Date: Tue, 11 Jun 2024 13:26:09 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: syzbot <syzbot+5f996b83575ef4058638@...kaller.appspotmail.com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
  linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [usb?] INFO: rcu detected stall in raw_ioctl

On Tue, Jun 11, 2024 at 08:53:02AM -0700, syzbot wrote:
> Hello,
> 
> syzbot has tested the proposed patch and the reproducer did not trigger any issue:
> 
> Reported-and-tested-by: syzbot+5f996b83575ef4058638@...kaller.appspotmail.com
> 
> Tested on:
> 
> commit:         8867bbd4 mm: arm64: Fix the out-of-bounds issue in con..
> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
> console output: https://syzkaller.appspot.com/x/log.txt?x=11bcfa36980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=3b4350cf56c61c80
> dashboard link: https://syzkaller.appspot.com/bug?extid=5f996b83575ef4058638
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> userspace arch: arm64
> patch:          https://syzkaller.appspot.com/x/patch.diff?x=17790922980000
> 
> Note: testing is done by a robot and is best-effort only.

The interval between adjacent timer interrupts is consistently 20 ms.  
Longer than expected, but it shouldn't be deadly.  Perhaps it would have 
been smaller if I hadn't pinned all the interrupts to the same CPU.

Anyway, let's see what happens with the dev_err() calls in the interrupt 
handler changed to dev_dbg().  That ought to reduce the overhead 
considerably.

Alan Stern

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git 8867bbd4a056

Index: usb-devel/drivers/usb/class/cdc-wdm.c
===================================================================
--- usb-devel.orig/drivers/usb/class/cdc-wdm.c
+++ usb-devel/drivers/usb/class/cdc-wdm.c
@@ -266,14 +266,14 @@ static void wdm_int_callback(struct urb
 			dev_err(&desc->intf->dev, "Stall on int endpoint\n");
 			goto sw; /* halt is cleared in work */
 		default:
-			dev_err(&desc->intf->dev,
+			dev_dbg(&desc->intf->dev,
 				"nonzero urb status received: %d\n", status);
 			break;
 		}
 	}
 
 	if (urb->actual_length < sizeof(struct usb_cdc_notification)) {
-		dev_err(&desc->intf->dev, "wdm_int_callback - %d bytes\n",
+		dev_dbg(&desc->intf->dev, "wdm_int_callback - %d bytes\n",
 			urb->actual_length);
 		goto exit;
 	}