Synchronize machine check panics with MCE broadcasts

For recent kernels before 2.6.31

Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
 arch/x86/kernel/cpu/mcheck/mce_64.c |   29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

Index: linux/arch/x86/kernel/cpu/mcheck/mce_64.c
===================================================================
--- linux.orig/arch/x86/kernel/cpu/mcheck/mce_64.c
+++ linux/arch/x86/kernel/cpu/mcheck/mce_64.c
@@ -147,6 +147,33 @@ static void print_mce(struct mce *m)
 	       "and contact your hardware vendor\n");
 }
 
+static void print_mce_panic(char *msg)
+{
+	static atomic_t mce_entry;
+	/*
+	 * Machine check panics often come up from multiple CPUs in parallel.
+	 * panic doesn't handle that well and deadlocks, so synchronize here.
+	 */
+	int first = atomic_add_return(1, &mce_entry) == 1;
+
+	if (!first) {
+		printk("CPU %d: MCE PANIC: %s\n", smp_processor_id(), msg);
+		/*
+		 * Enable interrupts so that smp_stop_cpus() can interrupts us,
+		 * but prevent scheduling in case someone compiles this
+		 * preemptible.
+		 * Then wait for the other panic to shut us down.
+		 */
+		preempt_disable();
+		local_irq_enable();
+		for (;;)
+			cpu_relax();
+	}
+
+	oops_in_progress = 1;
+	panic(msg);
+}
+
 static void mce_panic(char *msg, struct mce *backup, unsigned long start)
 {
 	int i;
@@ -163,7 +190,7 @@ static void mce_panic(char *msg, struct
 	}
 	if (backup)
 		print_mce(backup);
-	panic(msg);
+	print_mce_panic(msg);
 }
 
 int mce_available(struct cpuinfo_x86 *c)