printk/nmi: flush NMI messages on the system panic

In NMI context, printk() messages are stored into per-CPU buffers to
avoid a possible deadlock.  They are normally flushed to the main ring
buffer via an IRQ work.  But the work is never called when the system
calls panic() in the very same NMI handler.

This patch tries to flush NMI buffers before the crash dump is
generated.  In this case it does not risk a double release and bails out
when the logbuf_lock is already taken.  The aim is to get the messages
into the main ring buffer when possible.  It makes them better
accessible in the vmcore.

Then the patch tries to flush the buffers second time when other CPUs
are down.  It might be more aggressive and reset logbuf_lock.  The aim
is to get the messages available for the consequent kmsg_dump() and
console_flush_on_panic() calls.

The patch causes vprintk_emit() to be called even in NMI context again.
But it is done via printk_deferred() so that the console handling is
skipped.  Consoles use internal locks and we could not prevent a
deadlock easily.  They are explicitly called later when the crash dump
is not generated, see console_flush_on_panic().

Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: David Miller <davem@davemloft.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 51dd6b824fe2..f4da695fd615 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -127,11 +127,13 @@ extern void printk_nmi_init(void);
 extern void printk_nmi_enter(void);
 extern void printk_nmi_exit(void);
 extern void printk_nmi_flush(void);
+extern void printk_nmi_flush_on_panic(void);
 #else
 static inline void printk_nmi_init(void) { }
 static inline void printk_nmi_enter(void) { }
 static inline void printk_nmi_exit(void) { }
 static inline void printk_nmi_flush(void) { }
+static inline void printk_nmi_flush_on_panic(void) { }
 #endif /* PRINTK_NMI */
 
 #ifdef CONFIG_PRINTK