dump_stack: serialize the output from dump_stack()

Add functionality to serialize the output from dump_stack() to avoid
mangling of the output when dump_stack is called simultaneously from
multiple cpus.

[akpm@linux-foundation.org: fix comment indenting, avoid inclusion of <asm/> files - use <linux/> where possiblem fix uniprocessor build (__dump_stack undefined), remove unneeded ifdef around smp.h inclusion]
Signed-off-by: Alex Thorlton <athorlton@sgi.com>
Reported-by: Russ Anderson <rja@sgi.com>
Reviewed-by: Robin Holt <holt@sgi.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/dump_stack.c

@@ -6,15 +6,58 @@
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/sched.h>
+#include <linux/smp.h>
+#include <linux/atomic.h>
+
+static void __dump_stack(void)
+{
+	dump_stack_print_info(KERN_DEFAULT);
+	show_stack(NULL, NULL);
+}
 
 /**
  * dump_stack - dump the current task information and its stack trace
  *
  * Architectures can override this implementation by implementing its own.
  */
+#ifdef CONFIG_SMP
+static atomic_t dump_lock = ATOMIC_INIT(-1);
+
 void dump_stack(void)
 {
-	dump_stack_print_info(KERN_DEFAULT);
-	show_stack(NULL, NULL);
+	int was_locked;
+	int old;
+	int cpu;
+
+	/*
+	 * Permit this cpu to perform nested stack dumps while serialising
+	 * against other CPUs
+	 */
+	preempt_disable();
+
+retry:
+	cpu = smp_processor_id();
+	old = atomic_cmpxchg(&dump_lock, -1, cpu);
+	if (old == -1) {
+		was_locked = 0;
+	} else if (old == cpu) {
+		was_locked = 1;
+	} else {
+		cpu_relax();
+		goto retry;
+	}
+
+	__dump_stack();
+
+	if (!was_locked)
+		atomic_set(&dump_lock, -1);
+
+	preempt_enable();
+}
+#else
+void dump_stack(void)
+{
+	__dump_stack();
 }
+#endif
 EXPORT_SYMBOL(dump_stack);