NetBSD Problem Report #57102

From www@netbsd.org  Sat Dec 10 16:03:23 2022
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Date: Sat, 10 Dec 2022 16:03:20 +0000 (UTC)
From: cryintothebluesky@gmail.com
Reply-To: cryintothebluesky@gmail.com
To: gnats-bugs@NetBSD.org
Subject: vcmbox0 thread using 100% CPU
X-Send-Pr-Version: www-1.0

>Number:         57102
>Category:       port-arm
>Synopsis:       vcmbox0 thread using 100% CPU
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-arm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 10 16:05:00 +0000 2022
>Last-Modified:  Tue Dec 16 09:55:01 +0000 2025
>Originator:     Sad Clouds
>Release:        9.99.108
>Organization:
>Environment:
NetBSD rp4 9.99.108 NetBSD 9.99.108 (GENERIC64) #0: Fri Dec  9 13:52:51 GMT 2022
>Description:
The issue occurred on Raspberry Pi 4, used as NFS server. Initiated a large copy from Linux NFS client to NetBSD NFS server. Some time later, vcmbox0 thread was consuming 100% CPU and the following message was logged before I rebooted the system

Dec 10 15:01:13 rp4 /netbsd: [ 721.2070326] vcmbox0: workqueue busy: updates stopped

Running envstat on command line blocked indefinitely.

Reboot was the only way to resolve the issue. Looks like there may be a bug that results in infinite loop.
>How-To-Repeat:
Not sure, after reboot, subsequent NFS usage did not reproduce the issue.
>Fix:

>Audit-Trail:
From: Taylor R Campbell <riastradh@NetBSD.org>
To: cryintothebluesky@gmail.com
Cc: gnats-bugs@netbsd.org, port-arm-maintainer@netbsd.org,
	gnats-admin@netbsd.org, netbsd-bugs@netbsd.org
Subject: Re: port-arm/57102: vcmbox0 thread using 100% CPU
Date: Sat, 10 Dec 2022 18:58:37 +0000

 > Date: Sat, 10 Dec 2022 16:05:00 +0000 (UTC)
 > From: cryintothebluesky@gmail.com
 > 
 > The issue occurred on Raspberry Pi 4, used as NFS server. Initiated
 > a large copy from Linux NFS client to NetBSD NFS server. Some time
 > later, vcmbox0 thread was consuming 100% CPU and the following
 > message was logged before I rebooted the system

 If you catch this again, it would be helpful if you could start
 crash(8), run

 crash> ps

 to find the thread occupying the CPU, which will look something like
 this:

 10618>10618 7   1         0   ffffa06b1676e940                 sh

 Note the `>' on the line marking it as a running thread, and the
 thread name is not `idle/N'.  Most likely the thread name will be
 vcmbox0 or something.  Copy & paste the horrible number like
 ffffa06b1676e940 into bt/a:

 crash> bt/a ffffa06b1676e940

 Sharing the stack trace should help track down what it's up to.

 The full output of ps and ps/w might also be helpful, so it wouldn't
 hurt to include those too.

From: Sad Clouds <cryintothebluesky@gmail.com>
To: gnats-bugs@NetBSD.org
Cc: 
Subject: Re: port-arm/57102: vcmbox0 thread using 100% CPU
Date: Tue, 16 Dec 2025 09:50:36 +0000

 I'm not able to get back trace for some reason:

 crash> ps
 0    >  118 7   3     40200   ffff0000fa3e2100            vcmbox0

 crash> bt/a ffff0000fa3e2100
 trace: no trapframe found for lwp: 0xffff0000fa3e2100
 crash: kvm_read(0xffff00000000, 8): kvm_read: Bad address

 Also the when vcmbox0 is stuck 100% on CPU this lasts for a few
 minutes, then gradually drops to 0% usage and everything is back to
 normal. Not sure what can be attributed to this behavior.

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