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 21:07:49 +0000
From: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To: Chuck Lever III <chuck.lever@...cle.com>
CC: Neil Brown <neilb@...e.de>, 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 26/04/24 09:05, Chuck Lever III wrote:
>
>> On Apr 25, 2024, at 4:51 PM, Chris Packham <Chris.Packham@...iedtelesis.co.nz> wrote:
>>
>>
>> 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.
> I'm 2/3 of the way through testing 5.15.156 with 390390240145
> applied, so it would be just another day before I can send a
> patch to stable@.
>
> May I add Tested-by: Chris Packham <Chris.Packham@...iedtelesis.co.nz> ?

Sure go ahead.