proc/kcore: don't grab lock for memory hotplug notifier

The memory hotplug notifier kcore_callback() only needs kclist_lock to
prevent races with __kcore_update_ram(), but we can easily eliminate that
race by using an atomic xchg() in __kcore_update_ram().  This is
preparation for converting kclist_lock to an rwsem.

Link: http://lkml.kernel.org/r/0a4bc89f4dbde8b5b2ea309f7b4fb6a85fe29df2.1531953780.git.osandov@fb.com
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/kcore.c

@@ -118,7 +118,7 @@ static void __kcore_update_ram(struct list_head *list)
 	LIST_HEAD(garbage);
 
 	write_lock(&kclist_lock);
-	if (kcore_need_update) {
+	if (xchg(&kcore_need_update, 0)) {
 		list_for_each_entry_safe(pos, tmp, &kclist_head, list) {
 			if (pos->type == KCORE_RAM
 				|| pos->type == KCORE_VMEMMAP)
@@ -127,7 +127,6 @@ static void __kcore_update_ram(struct list_head *list)
 		list_splice_tail(list, &kclist_head);
 	} else
 		list_splice(list, &garbage);
-	kcore_need_update = 0;
 	proc_root_kcore->size = get_kcore_size(&nphdr, &size);
 	write_unlock(&kclist_lock);
 
@@ -593,9 +592,8 @@ static int __meminit kcore_callback(struct notifier_block *self,
 	switch (action) {
 	case MEM_ONLINE:
 	case MEM_OFFLINE:
-		write_lock(&kclist_lock);
 		kcore_need_update = 1;
-		write_unlock(&kclist_lock);
+		break;
 	}
 	return NOTIFY_OK;
 }