This is a note to let you know that I've just added the patch titled printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: printk-restore-flushing-of-nmi-buffers-on-remote-cpus-after-nmi-backtraces.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 5d5e4522a7f404d1a96fd6c703989d32a9c9568d Mon Sep 17 00:00:00 2001 From: Nicholas Piggin <npiggin@xxxxxxxxx> Date: Sun, 7 Nov 2021 14:51:16 +1000 Subject: printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces From: Nicholas Piggin <npiggin@xxxxxxxxx> commit 5d5e4522a7f404d1a96fd6c703989d32a9c9568d upstream. printk from NMI context relies on irq work being raised on the local CPU to print to console. This can be a problem if the NMI was raised by a lockup detector to print lockup stack and regs, because the CPU may not enable irqs (because it is locked up). Introduce printk_trigger_flush() that can be called another CPU to try to get those messages to the console, call that where printk_safe_flush was previously called. Fixes: 93d102f094be ("printk: remove safe buffers") Cc: stable@xxxxxxxxxxxxxxx # 5.15 Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx> Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx> Link: https://lore.kernel.org/r/20211107045116.1754411-1-npiggin@xxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/powerpc/kernel/watchdog.c | 6 ++++++ include/linux/printk.h | 4 ++++ kernel/printk/printk.c | 5 +++++ lib/nmi_backtrace.c | 6 ++++++ 4 files changed, 21 insertions(+) --- a/arch/powerpc/kernel/watchdog.c +++ b/arch/powerpc/kernel/watchdog.c @@ -187,6 +187,12 @@ static void watchdog_smp_panic(int cpu, if (sysctl_hardlockup_all_cpu_backtrace) trigger_allbutself_cpu_backtrace(); + /* + * Force flush any remote buffers that might be stuck in IRQ context + * and therefore could not run their irq_work. + */ + printk_trigger_flush(); + if (hardlockup_panic) nmi_panic(NULL, "Hard LOCKUP"); --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -198,6 +198,7 @@ void dump_stack_print_info(const char *l void show_regs_print_info(const char *log_lvl); extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold; extern asmlinkage void dump_stack(void) __cold; +void printk_trigger_flush(void); #else static inline __printf(1, 0) int vprintk(const char *s, va_list args) @@ -274,6 +275,9 @@ static inline void dump_stack_lvl(const static inline void dump_stack(void) { } +static inline void printk_trigger_flush(void) +{ +} #endif #ifdef CONFIG_SMP --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3252,6 +3252,11 @@ void defer_console_output(void) preempt_enable(); } +void printk_trigger_flush(void) +{ + defer_console_output(); +} + int vprintk_deferred(const char *fmt, va_list args) { int r; --- a/lib/nmi_backtrace.c +++ b/lib/nmi_backtrace.c @@ -75,6 +75,12 @@ void nmi_trigger_cpumask_backtrace(const touch_softlockup_watchdog(); } + /* + * Force flush any remote buffers that might be stuck in IRQ context + * and therefore could not run their irq_work. + */ + printk_trigger_flush(); + clear_bit_unlock(0, &backtrace_flag); put_cpu(); } Patches currently in stable-queue which might be from npiggin@xxxxxxxxx are queue-5.15/powerpc-pseries-fix-numa-form2-parsing-fallback-code.patch queue-5.15/printk-restore-flushing-of-nmi-buffers-on-remote-cpus-after-nmi-backtraces.patch queue-5.15/powerpc-pseries-rename-numa_dist_table-to-form2_dist.patch