printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces

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: 93d102f0 ("printk: remove safe buffers")
Cc: stable@vger.kernel.org # 5.15
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20211107045116.1754411-1-npiggin@gmail.com

arch/powerpc/kernel/watchdog.c

@@ -186,6 +186,12 @@ static void watchdog_smp_panic(int cpu, u64 tb)
 	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");
 

include/linux/printk.h

@@ -206,6 +206,7 @@ void dump_stack_print_info(const char *log_lvl);
 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)
@@ -282,6 +283,9 @@ static inline void dump_stack_lvl(const char *log_lvl)
 static inline void dump_stack(void)
 {
 }
+static inline void printk_trigger_flush(void)
+{
+}
 #endif
 
 #ifdef CONFIG_SMP

kernel/printk/printk.c

@@ -3261,6 +3261,11 @@ 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;

lib/nmi_backtrace.c

@@ -75,6 +75,12 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
 		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();
 }