[PATCH 0/2] panic: flush unsafe consoles before reboot and kdump

[Breno Leitao <leitao@debian.org>]

Since netconsole moved to NBCON, its messages are lost when the
machine reboots after a panic or boots a crash kernel. netconsole is
a CON_NBCON_ATOMIC_UNSAFE console, and those are only flushed at the
very end of panic(), after both.

Ryan reported this [1], and the fix is based on Petr's suggestions
[2][3]. Patch 1 flushes the unsafe consoles from printk() on the
panic CPU. Patch 2 also flushes them in __crash_kexec(), for an oops
that goes straight to kdump: on x86 that path prints nothing once the
CPU has become the panic CPU.

Tested on x86 with netconsole sending to a receiver: panic reboot,
kdump from sysrq and kdump from an oops all deliver the report, and
the crash kernel boots. Under virtme-ng that is 21 of 21 runs,
against 0 of 21 without the series. On a bare-metal host each case
was run once, and an unpatched kernel delivered nothing in all
three. arm64 is untested.

One case is left open. A printk() inside machine_crash_shutdown()
now also reaches netconsole. On x86 that only happens on errors, but
on a TDX guest tdx_kexec_finish() can pr_err() after it has unmapped
the shared memory a virtio-net netconsole uses. Stopping the printk()
flush once __crash_kexec() has flushed would close it, but would also
drop "Starting crashdump kernel..." and "Bye!" from netconsole on
arm64.

[1] https://lore.kernel.org/all/20260915100140.3631650-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/all/auNaD8LELWok3LOF@pathway/
[3] https://lore.kernel.org/all/auhl3cdwrHt4Vw7U@pathway/

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Breno Leitao (2):
      printk: nbcon: Flush unsafe consoles from printk() in panic
      crash: Flush unsafe consoles before machine_crash_shutdown()

 include/linux/console.h |  7 ++++---
 include/linux/printk.h  |  5 +++++
 kernel/crash_core.c     |  1 +
 kernel/printk/nbcon.c   | 38 ++++++++++++++++++++++++++++++++------
 4 files changed, 42 insertions(+), 9 deletions(-)
---
base-commit: a39c950806213f6861d00f90729acaaa21f2d95a
change-id: 20261030-panic_unsafe_flush-62939c2c28e7

Best regards,
--  
Breno Leitao <leitao@debian.org>