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: 93d102f094be ("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

diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index dc17d8903d4f375273b9ae5c51c1c55ff6e1c3e0..6b7a83d5e03edb7553e98e66960c4afd8dbc1961 100644 (file)
--- a/arch/powerpc/kernel/watchdog.c
+++ b/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");
 

diff --git a/include/linux/printk.h b/include/linux/printk.h
index a1379df43251862aea77d4a7cbacb5293abab413..596ad6fa03366847302b75212754c85547f3df76 100644 (file)
--- a/include/linux/printk.h
+++ b/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

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 65fffa6368c9388894edcf48f0950202731a1cb6..eabe23b0a982f6e6f3a75164311aa4f77b871800 100644 (file)
--- a/kernel/printk/printk.c
+++ b/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;

diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
index f9e89001b52ebdc08e3f5e6afa8c44fbf576844d..199ab201d5019c7c216f4db0f32da947e1d4503c 100644 (file)
--- a/lib/nmi_backtrace.c
+++ b/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();
 }