[PATCH v3] kdump: Fix crash_kexec - smp_send_stop race in panic

[Michael Holzheu <holzheu-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

From: Michael Holzheu <holzheu-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
Subject: kdump: Fix crash_kexec()/smp_send_stop() race in panic

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.

All other CPUs call the weak function panic_smp_self_stop() that stops
the CPU itself. This function can be overloaded by architecture code.
For example "tile" can use their lower-power "nap" instruction for
that.

Acked-by: Chris Metcalf <cmetcalf-kv+TWInifGbQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Michael Holzheu <holzheu-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
---
 kernel/panic.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -49,6 +49,15 @@ static long no_blink(int state)
 long (*panic_blink)(int state);
 EXPORT_SYMBOL(panic_blink);
 
+/*
+ * Stop ourself in panic -- architecture code may override this
+ */
+void __weak panic_smp_self_stop(void)
+{
+	while (1)
+		cpu_relax();
+}
+
 /**
  *	panic - halt the system
  *	@fmt: The text string to print
@@ -59,6 +68,7 @@ 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;
@@ -68,8 +78,14 @@ NORET_TYPE void panic(const char * fmt,
 	 * It's possible to come here directly from a panic-assertion and
 	 * not have preempt disabled. Some functions called from here want
 	 * preempt to be disabled. No point enabling it later though...
+	 *
+	 * Only one CPU is allowed to execute the panic code from here. For
+	 * multiple parallel invocations of panic, all other CPUs either
+	 * stop themself or will wait until they are stopped by the 1st CPU
+	 * with smp_send_stop().
 	 */
-	preempt_disable();
+	if (!spin_trylock(&panic_lock))
+		panic_smp_self_stop();
 
 	console_verbose();
 	bust_spinlocks(1);