I have now identified the lower-level trigger for the generator timeout.
During a controlled reproduction, `systemd-fstab-generator` was already
hanging before the package upgrade. Tracing showed that it blocks while
accessing an existing NFS mount:
fass-v:/pool -> /mnt/fass/pool
NFSv4.1, hard, sec=krb5
The underlying NFS client is stuck in NFSv4.1 session recovery against
Microsoft Server for NFS.
On affected clients we observed:
nfsv4 SEQUENCE ... q:xprt_pending
and, on one client, the NFS state-manager thread is blocked in:
nfs4_drain_slot_tbl
nfs4_reset_session
nfs4_state_manager
nfs4_run_state_manager
New requests wait on the ForeChannel slot table. TCP connectivity to the
server is still alive, but a 135-second packet capture showed no NFS RPC
progress.
This also reproduces independently on another Ubuntu 24.04 client with
systemd 255.4-1ubuntu8.17 and kernel 6.8.
During the fatal systemd reproduction the sequence is therefore now
clear:
hung NFSv4.1 mount
-> systemd-fstab-generator blocks
-> daemon-reexec
-> generator infrastructure times out after ~90 seconds
-> "Failed to fork off sandboxing environment for executing generators:
Protocol error"
-> "Freezing execution."
So the NFS problem is the trigger, but the systemd issue remains that a
blocked generator during `daemon-reexec` causes PID 1 to enter the fatal
"Freezing execution" state instead of recovering or failing the
generator operation gracefully.
I have also reported the underlying NFSv4.1 recovery hang to the
existing linux-nfs upstream thread:
https://www.spinics.net/lists/kernel/msg6188959.html
I can provide the generator strace, SUNRPC debugfs state, mountstats and
packet capture if useful.
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https://bugs.launchpad.net/bugs/2165295
Title:
systemd freezes after daemon-reexec triggered by unattended-upgrades /
needrestart on Ubuntu 24.04
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