On Tue, 2011-10-25 at 05:04 -0700, Eric W. Biederman wrote:
> Michael Holzheu <holzheu at linux.vnet.ibm.com> writes:
>
> > 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:
> Do we want both panic printks?
Ok, good point. We proably should not change that.
> We really only need the mutual exclusion starting just before
> smp_send_stop so that is where I would be inclined to put it.
I think to fix the race, at least we have the get the lock before we
call crash_kexec().
Is the following patch ok for you?
---
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?
