[PATCH 4/7] panic: Always flush printk buffer before panic

[Jan Kara <jack@suse.com>]

In some cases we may end up killing the CPU holding the console lock
while still having valuable data in logbuf. E.g. Vitaly is observing the
following:

- A crash is happening on one CPU and console_unlock() is being called
  on some other.

- console_unlock() tries to print out the buffer before releasing the
  lock and on slow console it takes time.

- in the meanwhile crashing CPU does lots of printk()-s with valuable
  data (which go to the logbuf) and sends IPIs to all other CPUs.

- console_unlock() finishes printing previous chunk and enables
  interrupts before trying to print out the rest, the CPU catches the
  IPI and never releases console lock.

This is not the only possible case: in VT/fb subsystems we have many
other console_lock()/console_unlock() users. Non-masked interrupts (or
receiving NMI in case of extreme slowness) will have the same result.

Getting the whole console buffer printed out on crash is top priority.
So zap printk locks and print logbuf contents after all cpus have been
stopped.

Based on patch by Vitaly Kuznetsov.

CC: Vitaly Kuznetsov <vkuznets@redhat.com>
Reported-and-tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Jan Kara <jack@suse.com>
---
 include/linux/console.h | 4 ++--
 kernel/panic.c          | 8 ++++++++
 kernel/printk/printk.c  | 5 ++++-
 kernel/stop_machine.c   | 2 +-
 4 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 96da462cdfeb..f40084802f3f 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -151,13 +151,13 @@ extern void console_unlock(void);
 extern void console_conditional_schedule(void);
 extern void console_unblank(void);
 #ifdef CONFIG_SMP
-extern void printk_log_buf_drain(void);
+extern void printk_log_buf_drain(bool panic);
 #else
 /*
  * In non-SMP kernels there won't be much to drain so save some code for tiny
  * kernels.
  */
-static inline void printk_log_buf_drain(void)
+static inline void printk_log_buf_drain(bool panic)
 {
 }
 #endif
diff --git a/kernel/panic.c b/kernel/panic.c
index 04e91ff7560b..d07ed830a9fb 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -23,6 +23,7 @@
 #include <linux/sysrq.h>
 #include <linux/init.h>
 #include <linux/nmi.h>
+#include <linux/console.h>
 
 #define PANIC_TIMER_STEP 100
 #define PANIC_BLINK_SPD 18
@@ -147,6 +148,13 @@ void panic(const char *fmt, ...)
 
 	bust_spinlocks(0);
 
+	/*
+	 * We may have ended up stopping the CPU doing printing (in
+	 * smp_send_stop()) while still having some valuable data in the
+	 * console buffer.  Flush it out.
+	 */
+	printk_log_buf_drain(true);
+
 	if (!panic_blink)
 		panic_blink = no_blink;
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8dc6c146d022..e404c429fe87 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2495,11 +2495,14 @@ struct tty_driver *console_device(int *index)
  * console. Note that as soon as this function returns, new messages may be
  * added to the printk buffer by other CPUs.
  */
-void printk_log_buf_drain(void)
+void printk_log_buf_drain(bool panic)
 {
 	bool retry;
 	unsigned long flags;
 
+	if (panic)
+		zap_locks();
+
 	while (1) {
 		raw_spin_lock_irqsave(&logbuf_lock, flags);
 		retry = console_seq != log_next_seq;
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index e9496b4a3825..50a03735893e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -550,7 +550,7 @@ static int __stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cp
 	 * finish thus triggering NMI watchdog, RCU lockups etc. Wait for the
 	 * printing here to avoid these.
 	 */
-	printk_log_buf_drain();
+	printk_log_buf_drain(false);
 
 	/* Set the initial state and stop all online cpus. */
 	set_state(&msdata, MULTI_STOP_PREPARE);
-- 
2.1.4