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

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Date: Thu, 25 Apr 2024 20:51:09 +0000
From: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To: NeilBrown <neilb@...e.de>, Chuck Lever III <chuck.lever@...cle.com>
CC: Jeff Layton <jlayton@...nel.org>, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>, Linux NFS Mailing List
	<linux-nfs@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: kernel BUG at net/sunrpc/svc.c:570 after updating from v5.15.153
 to v5.15.155


On 25/04/24 11:37, NeilBrown wrote:
> On Thu, 25 Apr 2024, Chuck Lever III wrote:
>>> On Apr 24, 2024, at 9:33 AM, Chuck Lever III <chuck.lever@...cle.com> wrote:
>>>
>>>> On Apr 24, 2024, at 3:42 AM, Chris Packham <Chris.Packham@...iedtelesis.co.nz> 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".
> I think this likely is the problem.  The nfsd threads must be being
> killed by a signal.
> One only other cause for an nfsd thread to exit is if
> svc_set_num_threads() is called, and all places that call that hold a
> ref on the serv structure so the final put won't happen when the thread
> exits.
>
> Before the patch that bisect found, the nfsd thread would exit with
>
>   svc_get();
>   svc_exit_thread();
>   nfsd_put();
>
> This also holds a ref across the svc_exit_thread(), and ensures the
> final 'put' happens from nfsD_put(), not svc_put() (in
> svc_exit_thread()).
>
> Chris: what was the context when the crash happened?  Could the nfsd
> threads have been signalled?  That hasn't been the standard way to stop
> nfsd threads for a long time, so I'm a little surprised that it is
> happening.

We use a hacked up version of shutdown from util-linux and which does a 
`kill (-1, SIGTERM);` then `kill (-1, SIGKILL);` (I don't think that 
particular behaviour is the hackery). I'm not sure if -1 will pick up 
kernel threads but based on the symptoms it appears to be doing so (or 
maybe something else is in it's SIGTERM handler). I don't think we were 
ever really intending to send the signals to nfsd so whether it actually 
terminates or not I don't think is an issue for us. I can confirm that 
applying 390390240145 resolves the symptom we were seeing.