Re: kernel BUG at net/sunrpc/svc.c:570 after updating from v5.15.153 to v5.15.155

[Chuck Lever III]

> On Apr 24, 2024, at 9:33 AM, Chuck Lever III <chuck.lever@xxxxxxxxxx> wrote:
> 
>> On Apr 24, 2024, at 3:42 AM, Chris Packham <Chris.Packham@xxxxxxxxxxxxxxxxxxx> wrote:
>> 
>> On 24/04/24 13:38, Chris Packham wrote:
>>> 
>>> On 24/04/24 12:54, Chris Packham wrote:
>>>> Hi Jeff, Chuck, Greg,
>>>> 
>>>> After updating one of our builds along the 5.15.y LTS branch our 
>>>> testing caught a new kernel bug. Output below.
>>>> 
>>>> I haven't dug into it yet but wondered if it rang any bells.
>>> 
>>> A bit more info. This is happening at "reboot" for us. Our embedded 
>>> devices use a bit of a hacked up reboot process so that they come back 
>>> faster in the case of a failure.
>>> 
>>> It doesn't happen with a proper `systemctl reboot` or with a SYSRQ+B
>>> 
>>> I can trigger it with `killall -9 nfsd` which I'm not sure is a 
>>> completely legit thing to do to kernel threads but it's probably close 
>>> to what our customized reboot does.
>> 
>> I've bisected between v5.15.153 and v5.15.155 and identified commit 
>> dec6b8bcac73 ("nfsd: Simplify code around svc_exit_thread() call in 
>> nfsd()") as the first bad commit. Based on the context that seems to 
>> line up with my reproduction. I'm wondering if perhaps something got 
>> missed out of the stable track? Unfortunately I'm not able to run a more 
>> recent kernel with all of the nfs related setup that is being used on  
>> the system in question.
> 
> Thanks for bisecting, that would have been my first suggestion.
> 
> The backport included all of the NFSD patches up to v6.2, but
> there might be a missing server-side SunRPC patch.

So dec6b8bcac73 ("nfsd: Simplify code around svc_exit_thread()
call in  nfsd()") is from v6.6, so it was applied to v5.15.y
only to get a subsequent NFSD fix to apply.

The immediately previous upstream commit is missing:

  390390240145 ("nfsd: don't allow nfsd threads to be signalled.")

For testing, I've applied this to my nfsd-5.15.y branch here:

  https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git

However even if that fixes the reported crash, this suggests
that after v6.6, nfsd threads are not going to respond to
"killall -9 nfsd".


--
Chuck Lever