GHES: Make NMI handler have a single reader

Since GHES sources are global, we theoretically need only a single CPU
reading them per NMI instead of a thundering herd of CPUs waiting on a
spinlock in NMI context for no reason at all.

Do that.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Borislav Petkov <bp@suse.de>

drivers/acpi/apei/ghes.c

@@ -729,10 +729,10 @@ static struct llist_head ghes_estatus_llist;
 static struct irq_work ghes_proc_irq_work;
 
 /*
- * NMI may be triggered on any CPU, so ghes_nmi_lock is used for
- * mutual exclusion.
+ * NMI may be triggered on any CPU, so ghes_in_nmi is used for
+ * having only one concurrent reader.
  */
-static DEFINE_RAW_SPINLOCK(ghes_nmi_lock);
+static atomic_t ghes_in_nmi = ATOMIC_INIT(0);
 
 static LIST_HEAD(ghes_nmi);
 
@@ -840,7 +840,9 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
 	struct ghes *ghes;
 	int sev, ret = NMI_DONE;
 
-	raw_spin_lock(&ghes_nmi_lock);
+	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
+		return ret;
+
 	list_for_each_entry_rcu(ghes, &ghes_nmi, list) {
 		if (ghes_read_estatus(ghes, 1)) {
 			ghes_clear_estatus(ghes);
@@ -863,7 +865,7 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
 #ifdef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
 	irq_work_queue(&ghes_proc_irq_work);
 #endif
-	raw_spin_unlock(&ghes_nmi_lock);
+	atomic_dec(&ghes_in_nmi);
 	return ret;
 }