[PATCH] x86_64: Notify user of MCE events.

x86_64 uses a userspace mce utility to decode MCEs, this patch will ensure
that the user is notified of MCE events being logged too.
Signed-off-by: Zwane Mwaikambo <zwane@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/mce.c

@@ -31,6 +31,8 @@ static int mce_dont_init;
 static int tolerant = 1;
 static int banks;
 static unsigned long bank[NR_BANKS] = { [0 ... NR_BANKS-1] = ~0UL };
+static unsigned long console_logged;
+static int notify_user;
 
 /*
  * Lockless MCE logging infrastructure.
@@ -68,6 +70,9 @@ void mce_log(struct mce *mce)
 	smp_wmb();
 	mcelog.entry[entry].finished = 1;
 	smp_wmb();
+
+	if (!test_and_set_bit(0, &console_logged))
+		notify_user = 1;
 }
 
 static void print_mce(struct mce *m)
@@ -252,6 +257,19 @@ static void mcheck_timer(void *data)
 {
 	on_each_cpu(mcheck_check_cpu, NULL, 1, 1);
 	schedule_delayed_work(&mcheck_work, check_interval * HZ);
+
+	/*
+	 * It's ok to read stale data here for notify_user and
+	 * console_logged as we'll simply get the updated versions
+	 * on the next mcheck_timer execution and atomic operations
+	 * on console_logged act as synchronization for notify_user
+	 * writes.
+	 */
+	if (notify_user && console_logged) {
+		notify_user = 0;
+		clear_bit(0, &console_logged);
+		printk(KERN_INFO "Machine check events logged\n");
+	}
 }