[PATCH] kdump: Fix crash_kexec - smp_send_stop race in panic

[Michael Holzheu]

Hello Andrew,

After the discussion with Eric and Vivek the following patch
seems to be a good solution to me. Could you accept this patch?

When two CPUs call panic at the same time there is a
possible race condition that can stop kdump. The first
CPU calls crash_kexec() and the second CPU calls
smp_send_stop() in panic() before crash_kexec() finished
on the first CPU. So the second CPU stops the first CPU
and therefore kdump fails:

1st CPU:
panic()->crash_kexec()->mutex_trylock(&kexec_mutex)-> do kdump

2nd CPU:
panic()->crash_kexec()->kexec_mutex already held by 1st CPU
       ->smp_send_stop()-> stop 1st CPU (stop kdump)

This patch fixes the problem by introducing a spinlock in
panic that allows only one CPU to process crash_kexec() and
the subsequent panic code.

Signed-off-by: Michael Holzheu <holzheu at linux.vnet.ibm.com>
---
 kernel/panic.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -59,6 +59,7 @@ EXPORT_SYMBOL(panic_blink);
  */
 NORET_TYPE void panic(const char * fmt, ...)
 {
+	static DEFINE_SPINLOCK(panic_lock);
 	static char buf[1024];
 	va_list args;
 	long i, i_next = 0;
@@ -82,6 +83,13 @@ NORET_TYPE void panic(const char * fmt,
 #endif
 
 	/*
+	 * Only one CPU is allowed to execute the panic code from here. For
+	 * multiple parallel invocations of panic all other CPUs will wait on
+	 * the panic_lock. They are stopped afterwards by smp_send_stop().
+	 */
+	spin_lock(&panic_lock);
+
+	/*
 	 * If we have crashed and we have a crash kernel loaded let it handle
 	 * everything else.
 	 * Do we want to call this before we try to display a message?