[PATCH] x86: Avoid intermixing cpu dump_stack output on multi-processor systems

[Russ Anderson <rja@sgi.com>]

When multiple cpus on a multi-processor system call dump_stack()
at the same time, the backtrace lines get intermixed, making 
the output worthless.  Add a lock so each cpu stack dump comes
out as a coherent set.

For example, when a multi-processor system is NMIed, all of the
cpus call dump_stack() at the same time, resulting in output for
all of cpus getting intermixed, making it impossible to tell what
any individual cpu was doing.  With this patch each cpu prints
its stack lines as a coherent set, so one can see what each cpu
was doing.

It has been tested on a 4069 cpu system.

Signed-off-by: Russ Anderson <rja@sgi.com>

---
 arch/x86/kernel/dumpstack.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/arch/x86/kernel/dumpstack.c
===================================================================
--- linux.orig/arch/x86/kernel/dumpstack.c	2012-05-03 14:31:13.602345805 -0500
+++ linux/arch/x86/kernel/dumpstack.c	2012-05-03 14:51:43.805197563 -0500
@@ -186,7 +186,9 @@ void dump_stack(void)
 {
 	unsigned long bp;
 	unsigned long stack;
+	static DEFINE_SPINLOCK(lock);	/* Serialise the printks */
 
+	spin_lock(&lock);
 	bp = stack_frame(current, NULL);
 	printk("Pid: %d, comm: %.20s %s %s %.*s\n",
 		current->pid, current->comm, print_tainted(),
@@ -194,6 +196,7 @@ void dump_stack(void)
 		(int)strcspn(init_utsname()->version, " "),
 		init_utsname()->version);
 	show_trace(NULL, NULL, &stack, bp);
+	spin_unlock(&lock);
 }
 EXPORT_SYMBOL(dump_stack);
 
-- 
Russ Anderson, OS RAS/Partitioning Project Lead  
SGI - Silicon Graphics Inc          rja@sgi.com