Re: [PATCH v2] printk/nbcon: Restore IRQ in atomic flush after each emitted record

[Petr Mladek <pmladek@suse.com>]

On Fri 2025-12-12 13:45:20, Petr Mladek wrote:
> The commit d5d399efff6577 ("printk/nbcon: Release nbcon consoles ownership
> in atomic flush after each emitted record") prevented stall of a CPU
> which lost nbcon console ownership because another CPU entered
> an emergency flush.
> 
> But there is still the problem that the CPU doing the emergency flush
> might cause a stall on its own.
> 
> Let's go even further and restore IRQ in the atomic flush after
> each emitted record.
> 
> It is not a complete solution. The interrupts and/or scheduling might
> still be blocked when the emergency atomic flush was called with
> IRQs and/or scheduling disabled. But it should remove the following
> lockup:
> 
>   mlx5_core 0000:03:00.0: Shutdown was called
>   kvm: exiting hardware virtualization
>   arm-smmu-v3 arm-smmu-v3.10.auto: CMD_SYNC timeout at 0x00000103 [hwprod 0x00000104, hwcons 0x00000102]
>   smp: csd: Detected non-responsive CSD lock (#1) on CPU#4, waiting 5000000032 ns for CPU#00 do_nothing (kernel/smp.c:1057)
>   smp:     csd: CSD lock (#1) unresponsive.
>   [...]
>   Call trace:
>   pl011_console_write_atomic (./arch/arm64/include/asm/vdso/processor.h:12 drivers/tty/serial/amba-pl011.c:2540) (P)
>   nbcon_emit_next_record (kernel/printk/nbcon.c:1049)
>   __nbcon_atomic_flush_pending_con (kernel/printk/nbcon.c:1517)
>   __nbcon_atomic_flush_pending.llvm.15488114865160659019 (./arch/arm64/include/asm/alternative-macros.h:254 ./arch/arm64/include/asm/cpufeature.h:808 ./arch/arm64/include/asm/irqflags.h:192 kernel/printk/nbcon.c:1562 kernel/printk/nbcon.c:1612)
>   nbcon_atomic_flush_pending (kernel/printk/nbcon.c:1629)
>   printk_kthreads_shutdown (kernel/printk/printk.c:?)
>   syscore_shutdown (drivers/base/syscore.c:120)
>   kernel_kexec (kernel/kexec_core.c:1045)
>   __arm64_sys_reboot (kernel/reboot.c:794 kernel/reboot.c:722 kernel/reboot.c:722)
>   invoke_syscall (arch/arm64/kernel/syscall.c:50)
>   el0_svc_common.llvm.14158405452757855239 (arch/arm64/kernel/syscall.c:?)
>   do_el0_svc (arch/arm64/kernel/syscall.c:152)
>   el0_svc (./arch/arm64/include/asm/alternative-macros.h:254 ./arch/arm64/include/asm/cpufeature.h:808 ./arch/arm64/include/asm/irqflags.h:73 arch/arm64/kernel/entry-common.c:169 arch/arm64/kernel/entry-common.c:182 arch/arm64/kernel/entry-common.c:749)
>   el0t_64_sync_handler (arch/arm64/kernel/entry-common.c:820)
>   el0t_64_sync (arch/arm64/kernel/entry.S:600)
> 
> In this case, nbcon_atomic_flush_pending() is called from
> printk_kthreads_shutdown() with IRQs and scheduling enabled.
> 
> Note that __nbcon_atomic_flush_pending_con() is directly called also from
> nbcon_device_release() where the disabled IRQs might break PREEMPT_RT
> guarantees. But the atomic flush is called only in emergency or panic
> situations where the latencies are irrelevant anyway.
> 
> An ultimate solution would be a touching of watchdogs. But it would hide
> all problems. Let's do it later when anyone reports a stall which does
> not have a better solution.
> 
> Closes: https://lore.kernel.org/r/sqwajvt7utnt463tzxgwu2yctyn5m6bjwrslsnupfexeml6hkd@v6sqmpbu3vvu
> Tested-by: Breno Leitao <leitao@debian.org>
> Signed-off-by: Petr Mladek <pmladek@suse.com>

JFYI, the patch has been committed into printk/linux.git,
branch rework/atomic-flush-softlockup.

It is fixing a real-life problem. I am going to give it few days
in linux-next and crete pull request by the end of this week
or at Jan 2, 2026 (before or after my Christmass vacation).

Best Regards,
Petr