[PATCH printk 0/6] Introduce policies for unsafe flushing

[PATCH printk 0/6] Introduce policies for unsafe flushing

[John Ogness]

Until now unsafe flushing is only performed at the very end of
panic(), before entering the infinite loop. For other situations
(such as panic-kexec or panic-reboot or emergency-reboot) no
unsafe flushing is performed. The reasoning for this is because
an unsafe flush could lead to fireworks, which could prevent the
kexec or reboot from occurring.

However, some users may be willing to take on such risks in order
to see panic output. Indeed, for the netconsole (which does not
support safe atomic printing) it is a regression that such unsafe
printing does not occur. [0] 

This series adds unsafe flushing policy flags per-console to
allow individual consoles to perform unsafe flushing for such
scenarios. The unsafe flushing is hard-code-enabled for the
netconsole in patch 6/6. The justification is that this is not
less safe than the legacy netconsole.

Later these policy flags could be easily exposed to userspace
via /sys/class/consoles/, once the sysfs per-console interface
is available.

John Ogness

[0] https://lore.kernel.org/lkml/20260915100140.3631650-1-ryan.roberts@arm.com

John Ogness (6):
  printk: Introduce and specify flush reasons
  console: Introduce unsafe flush policy flags
  printk: Evaluate flush policy in nbcon_allow_unsafe_takeover()
  printk: Remove nbcon_atomic_flush_unsafe()
  printk: Add panic+kexec flush point
  netconsole: Always perform unsafe flushing on panic/kdb/restart

 arch/powerpc/kernel/watchdog.c |   2 +-
 drivers/net/netconsole.c       |   4 ++
 include/linux/console.h        |  37 ++++++++---
 include/linux/printk.h         |  27 ++++++--
 kernel/crash_core.c            |   1 +
 kernel/panic.c                 |   2 +-
 kernel/printk/internal.h       |   4 +-
 kernel/printk/nbcon.c          | 117 ++++++++++++++++++++++-----------
 kernel/printk/printk.c         |  16 ++---
 kernel/reboot.c                |   2 +-
 lib/nmi_backtrace.c            |   2 +-
 11 files changed, 146 insertions(+), 68 deletions(-)


base-commit: d01f86a7da8161aac8c6304b377cf58fb75861ca
-- 
2.47.3

[PATCH printk 5/6] printk: Add panic+kexec flush point

[John Ogness]

During panic, nbcon consoles transition to atomic printing. However,
there are consoles that do not support safe atomic printing
(CON_NBCON_ATOMIC_UNSAFE). Such consoles will only flush during
panic if their unsafe flush policy allows them to do so.

The unsafe flush policy NBCON_PANIC_FLUSH only allows unsafe
flushing when particular flushes are explicitly invoked. Call
printk_trigger_flush(PR_FLUSH_RESTART) before performing the kexec
in order to give such consoles a chance to perform an unsafe flush.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/crash_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index d0bd2d0cf8990..064b0af903c38 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -140,6 +140,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
 
 			crash_setup_regs(&fixed_regs, regs);
 			crash_save_vmcoreinfo();
+			printk_trigger_flush(PR_FLUSH_RESTART);
 			machine_crash_shutdown(&fixed_regs);
 			crash_cma_clear_pending_dma();
 			machine_kexec(kexec_crash_image);
-- 
2.47.3

Re: [PATCH printk 5/6] printk: Add panic+kexec flush point

[sashiko-bot]

> During panic, nbcon consoles transition to atomic printing. However,
> there are consoles that do not support safe atomic printing
> (CON_NBCON_ATOMIC_UNSAFE). Such consoles will only flush during
> panic if their unsafe flush policy allows them to do so.
> 
> The unsafe flush policy NBCON_PANIC_FLUSH only allows unsafe
> flushing when particular flushes are explicitly invoked. Call
> printk_trigger_flush(PR_FLUSH_RESTART) before performing the kexec
> in order to give such consoles a chance to perform an unsafe flush.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261109123127.489676-1-john.ogness@linutronix.de?part=5