Re: nfsd blocks indefinitely in nfsd4_destroy_session

Christian Herzog
2025-01-09T11:56:47Z

Dear Chuck,

On Wed, Jan 08, 2025 at 10:07:49AM -0500, Chuck Lever wrote:
> On 1/8/25 9:54 AM, Christian Herzog wrote:
> > Dear Chuck,
> >
> > On Wed, Jan 08, 2025 at 08:33:23AM -0500, Chuck Lever wrote:
> > > On 1/7/25 4:17 PM, Salvatore Bonaccorso wrote:
> > > > Hi Chuck,
> > > >
> > > > Thanks for your time on this, much appreciated.
> > > >
> > > > On Wed, Jan 01, 2025 at 02:24:50PM -0500, Chuck Lever wrote:
> > > > > On 12/25/24 4:15 AM, Salvatore Bonaccorso wrote:
> > > > > > Hi Chuck, hi all,
> > > > > >
> > > > > > [it was not ideal to pick one of the message for this followup, let me
> > > > > > know if you want a complete new thread, adding as well Benjamin and
> > > > > > Trond as they are involved in one mentioned patch]
> > > > > >
> > > > > > On Mon, Jun 17, 2024 at 02:31:54PM +0000, Chuck Lever III wrote:
> > > > > > >
> > > > > > >
> > > > > > > > On Jun 17, 2024, at 2:55 AM, Harald Dunkel <harald.dunkel@aixigo.com> wrote:
> > > > > > > >
> > > > > > > > Hi folks,
> > > > > > > >
> > > > > > > > what would be the reason for nfsd getting stuck somehow and becoming
> > > > > > > > an unkillable process? See
> > > > > > > >
> > > > > > > > - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1071562
> > > > > > > > - https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/2062568
> > > > > > > >
> > > > > > > > Doesn't this mean that something inside the kernel gets stuck as
> > > > > > > > well? Seems odd to me.
> > > > > > >
> > > > > > > I'm not familiar with the Debian or Ubuntu kernel packages. Can
> > > > > > > the kernel release numbers be translated to LTS kernel releases
> > > > > > > please? Need both "last known working" and "first broken" releases.
> > > > > > >
> > > > > > > This:
> > > > > > >
> > > > > > > [ 6596.911785] RPC: Could not send backchannel reply error: -110
> > > > > > > [ 6596.972490] RPC: Could not send backchannel reply error: -110
> > > > > > > [ 6837.281307] RPC: Could not send backchannel reply error: -110
> > > > > > >
> > > > > > > is a known set of client backchannel bugs. Knowing the LTS kernel
> > > > > > > releases (see above) will help us figure out what needs to be
> > > > > > > backported to the LTS kernels kernels in question.
> > > > > > >
> > > > > > > This:
> > > > > > >
> > > > > > > [11183.290619] wait_for_completion+0x88/0x150
> > > > > > > [11183.290623] __flush_workqueue+0x140/0x3e0
> > > > > > > [11183.290629] nfsd4_probe_callback_sync+0x1a/0x30 [nfsd]
> > > > > > > [11183.290689] nfsd4_destroy_session+0x186/0x260 [nfsd]
> > > > > > >
> > > > > > > is probably related to the backchannel errors on the client, but
> > > > > > > client bugs shouldn't cause the server to hang like this. We
> > > > > > > might be able to say more if you can provide the kernel release
> > > > > > > translations (see above).
> > > > > >
> > > > > > In Debian we hstill have the bug #1071562 open and one person notified
> > > > > > mye offlist that it appears that the issue get more frequent since
> > > > > > they updated on NFS client side from Ubuntu 20.04 to Debian bookworm
> > > > > > with a 6.1.y based kernel).
> > > > > >
> > > > > > Some people around those issues, seem to claim that the change
> > > > > > mentioned in
> > > > > > https://lists.proxmox.com/pipermail/pve-devel/2024-July/064614.html
> > > > > > would fix the issue, which is as well backchannel related.
> > > > > >
> > > > > > This is upstream: 6ddc9deacc13 ("SUNRPC: Fix backchannel reply,
> > > > > > again"). While this commit fixes 57331a59ac0d ("NFSv4.1: Use the
> > > > > > nfs_client's rpc timeouts for backchannel") this is not something
> > > > > > which goes back to 6.1.y, could it be possible that hte backchannel
> > > > > > refactoring and this final fix indeeds fixes the issue?
> > > > > >
> > > > > > As people report it is not easily reproducible, so this makes it
> > > > > > harder to identify fixes correctly.
> > > > > >
> > > > > > I gave a (short) stance on trying to backport commits up to
> > > > > > 6ddc9deacc13 ("SUNRPC: Fix backchannel reply, again") but this quickly
> > > > > > seems to indicate it is probably still not the right thing for
> > > > > > backporting to the older stable series.
> > > > > >
> > > > > > As at least pre-requisites:
> > > > > >
> > > > > > 2009e32997ed568a305cf9bc7bf27d22e0f6ccda
> > > > > > 4119bd0306652776cb0b7caa3aea5b2a93aecb89
> > > > > > 163cdfca341b76c958567ae0966bd3575c5c6192
> > > > > > f4afc8fead386c81fda2593ad6162271d26667f8
> > > > > > 6ed8cdf967f7e9fc96cd1c129719ef99db2f9afc
> > > > > > 57331a59ac0d680f606403eb24edd3c35aecba31
> > > > > >
> > > > > > and still there would be conflicting codepaths (and does not seem
> > > > > > right).
> > > > > >
> > > > > > Chuck, Benjamin, Trond, is there anything we can provive on reporters
> > > > > > side that we can try to tackle this issue better?
> > > > >
> > > > > As I've indicated before, NFSD should not block no matter what the
> > > > > client may or may not be doing. I'd like to focus on the server first.
> > > > >
> > > > > What is the result of:
> > > > >
> > > > > $ cd <Bookworm's v6.1.90 kernel source >
> > > > > $ unset KBUILD_OUTPUT
> > > > > $ make -j `nproc`
> > > > > $ scripts/faddr2line \
> > > > > fs/nfsd/nfs4state.o \
> > > > > nfsd4_destroy_session+0x16d
> > > > >
> > > > > Since this issue appeared after v6.1.1, is it possible to bisect
> > > > > between v6.1.1 and v6.1.90 ?
> > > >
> > > > First please note, at least speaking of triggering the issue in
> > > > Debian, Debian has moved to 6.1.119 based kernel already (and soon in
> > > > the weekends point release move to 6.1.123).
> > > >
> > > > That said, one of the users which regularly seems to be hit by the
> > > > issue was able to provide the above requested information, based for
> > > > 6.1.119:
> > > >
> > > > ~/kernel/linux-source-6.1# make kernelversion
> > > > 6.1.119
> > > > ~/kernel/linux-source-6.1# scripts/faddr2line fs/nfsd/nfs4state.o nfsd4_destroy_session+0x16d
> > > > nfsd4_destroy_session+0x16d/0x250:
> > > > __list_del_entry at /root/kernel/linux-source-6.1/./include/linux/list.h:134
> > > > (inlined by) list_del at /root/kernel/linux-source-6.1/./include/linux/list.h:148
> > > > (inlined by) unhash_session at /root/kernel/linux-source-6.1/fs/nfsd/nfs4state.c:2062
> > > > (inlined by) nfsd4_destroy_session at /root/kernel/linux-source-6.1/fs/nfsd/nfs4state.c:3856
> > > >
> > > > They could provide as well a decode_stacktrace output for the recent
> > > > hit (if that is helpful for you):
> > > >
> > > > [Mon Jan 6 13:25:28 2025] INFO: task nfsd:55306 blocked for more than 6883 seconds.
> > > > [Mon Jan 6 13:25:28 2025] Not tainted 6.1.0-28-amd64 #1 Debian 6.1.119-1
> > > > [Mon Jan 6 13:25:28 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > > [Mon Jan 6 13:25:28 2025] task:nfsd state:D stack:0 pid:55306 ppid:2 flags:0x00004000
> > > > [Mon Jan 6 13:25:28 2025] Call Trace:
> > > > [Mon Jan 6 13:25:28 2025] <TASK>
> > > > [Mon Jan 6 13:25:28 2025] __schedule+0x34d/0x9e0
> > > > [Mon Jan 6 13:25:28 2025] schedule+0x5a/0xd0
> > > > [Mon Jan 6 13:25:28 2025] schedule_timeout+0x118/0x150
> > > > [Mon Jan 6 13:25:28 2025] wait_for_completion+0x86/0x160
> > > > [Mon Jan 6 13:25:28 2025] __flush_workqueue+0x152/0x420
> > > > [Mon Jan 6 13:25:28 2025] nfsd4_destroy_session (debian/build/build_amd64_none_amd64/include/linux/spinlock.h:351 debian/build/build_amd64_none_amd64/fs/nfsd/nfs4state.c:3861) nfsd
> > > > [Mon Jan 6 13:25:28 2025] nfsd4_proc_compound (debian/build/build_amd64_none_amd64/fs/nfsd/nfs4proc.c:2680) nfsd
> > > > [Mon Jan 6 13:25:28 2025] nfsd_dispatch (debian/build/build_amd64_none_amd64/fs/nfsd/nfssvc.c:1022) nfsd
> > > > [Mon Jan 6 13:25:28 2025] svc_process_common (debian/build/build_amd64_none_amd64/net/sunrpc/svc.c:1344) sunrpc
> > > > [Mon Jan 6 13:25:28 2025] ? svc_recv (debian/build/build_amd64_none_amd64/net/sunrpc/svc_xprt.c:897) sunrpc
> > > > [Mon Jan 6 13:25:28 2025] ? nfsd_svc (debian/build/build_amd64_none_amd64/fs/nfsd/nfssvc.c:983) nfsd
> > > > [Mon Jan 6 13:25:28 2025] ? nfsd_inet6addr_event (debian/build/build_amd64_none_amd64/fs/nfsd/nfssvc.c:922) nfsd
> > > > [Mon Jan 6 13:25:28 2025] svc_process (debian/build/build_amd64_none_amd64/net/sunrpc/svc.c:1474) sunrpc
> > > > [Mon Jan 6 13:25:28 2025] nfsd (debian/build/build_amd64_none_amd64/fs/nfsd/nfssvc.c:960) nfsd
> > > > [Mon Jan 6 13:25:28 2025] kthread+0xd7/0x100
> > > > [Mon Jan 6 13:25:28 2025] ? kthread_complete_and_exit+0x20/0x20
> > > > [Mon Jan 6 13:25:28 2025] ret_from_fork+0x1f/0x30
> > > > [Mon Jan 6 13:25:28 2025] </TASK>
> > > >
> > > > The question about bisection is actually harder, those are production
> > > > systems and I understand it's not possible to do bisect there, while
> > > > unfortunately not reprodcing the issue on "lab conditions".
> > > >
> > > > Does the above give us still a clue on what you were looking for?
> > >
> > > Thanks for the update.
> > >
> > > It's possible that 38f080f3cd19 ("NFSD: Move callback_wq into struct
> > > nfs4_client"), while not a specific fix for this issue, might offer some
> > > relief by preventing the DESTROY_SESSION hang from affecting all other
> > > clients of the degraded server.
> > >
> > > Not having a reproducer or the ability to bisect puts a damper on
> > > things. The next step, then, is to enable tracing on servers where this
> > > issue can come up, and wait for the hang to occur. The following command
> > > captures information only about callback operation, so it should not
> > > generate much data or impact server performance.
> > >
> > > # trace-cmd record -e nfsd:nfsd_cb\*
> > >
> > > Let that run until the problem occurs, then ^C, compress the resulting
> > > trace.dat file, and either attach it to 1071562 or email it to me
> > > privately.
> > thanks for the follow-up.
> >
> > I am the "customer" with two affected file servers. We have since moved those
> > servers to the backports kernel (6.11.10) in the hope of forward fixing the
> > issue. If this kernel is stable, I'm afraid I can't go back to the 'bad'
> > kernel (700+ researchers affected..), and we're also not able to trigger the
> > issue on our test environment.
>
> Hello Dr. Herzog -
>
> If the problem recurs on 6.11, the trace-cmd I suggest above works
> there as well.
the bad news is: it just happened again with the bpo kernel.

We then immediately started trace-cmd since usually there are several call
traces in a row and we did get a trace.dat:
http://people.phys.ethz.ch/~daduke/trace.dat

we did notice however that the stack trace looked a bit different this time:

INFO: task nfsd:2566 blocked for more than 5799 seconds.
Tainted: G W 6.11.10+bpo-amd64 #1 Debia>
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables t>
task:nfsd state:D stack:0 pid:2566 tgid:2566 >
Call Trace:
<TASK>
__schedule+0x400/0xad0
schedule+0x27/0xf0
nfsd4_shutdown_callback+0xfe/0x140 [nfsd]
? __pfx_var_wake_function+0x10/0x10
__destroy_client+0x1f0/0x290 [nfsd]
nfsd4_destroy_clientid+0xf1/0x1e0 [nfsd]
? svcauth_unix_set_client+0x586/0x5f0 [sunrpc]
nfsd4_proc_compound+0x34d/0x670 [nfsd]
nfsd_dispatch+0xfd/0x220 [nfsd]
svc_process_common+0x2f7/0x700 [sunrpc]
? __pfx_nfsd_dispatch+0x10/0x10 [nfsd]
svc_process+0x131/0x180 [sunrpc]
svc_recv+0x830/0xa10 [sunrpc]
? __pfx_nfsd+0x10/0x10 [nfsd]
nfsd+0x87/0xf0 [nfsd]
kthread+0xcf/0x100
? __pfx_kthread+0x10/0x10
ret_from_fork+0x31/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>

and also the state of the offending client in `/proc/fs/nfsd/clients/*/info`
used to be callback state: UNKNOWN while now it is DOWN or FAULT. No idea
what it means, but I thought it was worth mentioning.

thanks!
-Christian

--
Dr. Christian Herzog <herzog@phys.ethz.ch> support: +41 44 633 26 68
Head, IT Services Group, HPT H 8 voice: +41 44 633 39 50
Department of Physics, ETH Zurich
8093 Zurich, Switzerland http://isg.phys.ethz.ch/