Re: [PATCH] printk/nmi: restore printk_func in nmi_panic

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

On (02/27/16 11:19), Sergey Senozhatsky wrote:
[..]
> > I think about a compromise. We should try to get the messages
> > out only when kdump is not enabled.
> 
> can we zap_locks() if we are on nmi_panic()->panic()->console_flush_on_panic() path?
> console_flush_on_panic() is happening after we send out smp_send_stop().

can something like this do the trick?

====8<====

>From 4186873bb4574b4bbb227e7448d56599849de0bd Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: [PATCH] printk/nmi: restore printk_func in nmi_panic

When watchdog detects a hardlockup and calls nmi_panic() `printk_func'
must be restored via printk_nmi_exit() call, so panic() will be able
to flush nmi buf and show backtrace and panic message. We also better
explicitly ask nmi to printk_nmi_flush() in console_flush_on_panic(),
because it may be too late to rely on irq work.

Factor out __zap_locks(), and call it from console_flush_on_panic().
This is normally not needed, because logbuf_lock always comes with
IRQ disable/enable magic, but we can panic() from nmi.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 include/linux/kernel.h |  6 ++++--
 kernel/printk/printk.c | 27 ++++++++++++++++-----------
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index f4fa2b2..3ee33d5 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -469,10 +469,12 @@ do {									\
 	cpu = raw_smp_processor_id();					\
 	old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, cpu);	\
 									\
-	if (old_cpu == PANIC_CPU_INVALID)				\
+	if (old_cpu == PANIC_CPU_INVALID) {				\
+		printk_nmi_exit();					\
 		panic(fmt, ##__VA_ARGS__);				\
-	else if (old_cpu != cpu)					\
+	} else if (old_cpu != cpu) {					\
 		nmi_panic_self_stop(regs);				\
+	}								\
 } while (0)
 
 /*
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9917f69..0a318ed 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1462,6 +1462,15 @@ static void call_console_drivers(int level,
 	}
 }
 
+static void __zap_locks(void)
+{
+	debug_locks_off();
+	/* If a crash is occurring, make sure we can't deadlock */
+	raw_spin_lock_init(&logbuf_lock);
+	/* And make sure that we print immediately */
+	sema_init(&console_sem, 1);
+}
+
 /*
  * Zap console related locks when oopsing.
  * To leave time for slow consoles to print a full oops,
@@ -1477,11 +1486,7 @@ static void zap_locks(void)
 
 	oops_timestamp = jiffies;
 
-	debug_locks_off();
-	/* If a crash is occurring, make sure we can't deadlock */
-	raw_spin_lock_init(&logbuf_lock);
-	/* And make sure that we print immediately */
-	sema_init(&console_sem, 1);
+	__zap_locks();
 }
 
 int printk_delay_msec __read_mostly;
@@ -2386,15 +2391,15 @@ void console_unblank(void)
  */
 void console_flush_on_panic(void)
 {
-	/*
-	 * If someone else is holding the console lock, trylock will fail
-	 * and may_schedule may be set.  Ignore and proceed to unlock so
-	 * that messages are flushed out.  As this can be called from any
-	 * context and we don't want to get preempted while flushing,
-	 * ensure may_schedule is cleared.
+	__zap_locks();
+
+	/* As this can be called from any context and we don't want
+	 * to get preempted while flushing, ensure may_schedule is
+	 * cleared.
 	 */
 	console_trylock();
 	console_may_schedule = 0;
+	printk_nmi_flush();
 	console_unlock();
 }
 
-- 
2.7.1