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@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".
> 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.
