pid: tighten pidmap spinlock critical section by removing kfree()

Avoid calling kfree() under pidmap spinlock, calling it afterwards.

Normally kfree() is fast, but sometimes it can be slow, so avoid
calling it under the spinlock if we can do it.
Signed-off-by: NAndré Goddard Rosa <andre.goddard@gmail.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/pid.c

@@ -141,11 +141,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			 * installing it:
 			 */
 			spin_lock_irq(&pidmap_lock);
-			if (map->page)
-				kfree(page);
-			else
+			if (!map->page) {
 				map->page = page;
+				page = NULL;
+			}
 			spin_unlock_irq(&pidmap_lock);
+			kfree(page);
 			if (unlikely(!map->page))
 				break;
 		}