Re: kdump: crash_kexec()-smp_send_stop() race in panic

[Michael Holzheu <holzheu@linux.vnet.ibm.com>]

Hello Eric,

On Mon, 2011-10-24 at 10:07 -0700, Eric W. Biederman wrote:

[snip]

> So my second thought is to introduce another atomic variable
> panic_in_progress, visible only in panic.  The cpu that sets
> increments panic_in_progress can call smp_send_stop.  The rest of
> the cpus can just go into a busy wait.  That should stop nasty
> fights about who is going to come out of smp_send_stop first.

So this is a spinlock, no? What about the following patch:
---
 kernel/panic.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- 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;
@@ -68,8 +69,12 @@ 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. For multiple
+	 * parallel invocations of panic all other CPUs will wait on the
+	 * panic_lock. They are stopped afterwards by smp_send_stop().
 	 */
-	preempt_disable();
+	spin_lock(&panic_lock);
 
 	console_verbose();
 	bust_spinlocks(1);