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From: "NeilBrown" <neilb@suse.de>
To: "Chuck Lever III" <chuck.lever@oracle.com>
Cc: "Chris Packham" <Chris.Packham@alliedtelesis.co.nz>,
 "Jeff Layton" <jlayton@kernel.org>,
 "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
 "Linux NFS Mailing List" <linux-nfs@vger.kernel.org>,
 "netdev" <netdev@vger.kernel.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: kernel BUG at net/sunrpc/svc.c:570 after updating from v5.15.153
 to v5.15.155
In-reply-to: <5D19EAF8-0F65-4CD6-9378-67234D407B96@oracle.com>
References: <>, <5D19EAF8-0F65-4CD6-9378-67234D407B96@oracle.com>
Date: Thu, 25 Apr 2024 09:37:31 +1000
Message-id: <171400185158.7600.16163546434537681088@noble.neil.brown.name>

On Thu, 25 Apr 2024, Chuck Lever III wrote:
>=20
> > On Apr 24, 2024, at 9:33=E2=80=AFAM, Chuck Lever III <chuck.lever@oracle.=
com> wrote:
> >=20
> >> On Apr 24, 2024, at 3:42=E2=80=AFAM, Chris Packham <Chris.Packham@allied=
telesis.co.nz> wrote:
> >>=20
> >> On 24/04/24 13:38, Chris Packham wrote:
> >>>=20
> >>> On 24/04/24 12:54, Chris Packham wrote:
> >>>> Hi Jeff, Chuck, Greg,
> >>>>=20
> >>>> After updating one of our builds along the 5.15.y LTS branch our=20
> >>>> testing caught a new kernel bug. Output below.
> >>>>=20
> >>>> I haven't dug into it yet but wondered if it rang any bells.
> >>>=20
> >>> A bit more info. This is happening at "reboot" for us. Our embedded=20
> >>> devices use a bit of a hacked up reboot process so that they come back =

> >>> faster in the case of a failure.
> >>>=20
> >>> It doesn't happen with a proper `systemctl reboot` or with a SYSRQ+B
> >>>=20
> >>> I can trigger it with `killall -9 nfsd` which I'm not sure is a=20
> >>> completely legit thing to do to kernel threads but it's probably close =

> >>> to what our customized reboot does.
> >>=20
> >> I've bisected between v5.15.153 and v5.15.155 and identified commit=20
> >> dec6b8bcac73 ("nfsd: Simplify code around svc_exit_thread() call in=20
> >> nfsd()") as the first bad commit. Based on the context that seems to=20
> >> line up with my reproduction. I'm wondering if perhaps something got=20
> >> missed out of the stable track? Unfortunately I'm not able to run a more=
=20
> >> recent kernel with all of the nfs related setup that is being used on =20
> >> the system in question.
> >=20
> > Thanks for bisecting, that would have been my first suggestion.
> >=20
> > The backport included all of the NFSD patches up to v6.2, but
> > there might be a missing server-side SunRPC patch.
>=20
> 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.
>=20
> The immediately previous upstream commit is missing:
>=20
>   390390240145 ("nfsd: don't allow nfsd threads to be signalled.")
>=20
> For testing, I've applied this to my nfsd-5.15.y branch here:
>=20
>   https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git
>=20
> 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.

NeilBrown