lib/plist.c: enforce memory ordering in plist_check_list

There exists an iteration over a plist in plist_check_list(), and memory
dependency exists between variables "prev", "next" and "prev->next". As
plist is used in the scheduling subsystem, we should guarantee the memory
ordering between multiple processors.

Using macro "WRITE_ONCE()" can help us to ensure the memory ordering as
it was stated in "Documentation/memory-barriers.txt".

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: I Hsin Cheng <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/plist.c

···
 
 	plist_check_prev_next(top, prev, next);
 	while (next != top) {
-		prev = next;
-		next = prev->next;
+		WRITE_ONCE(prev, next);
+		WRITE_ONCE(next, prev->next);
 		plist_check_prev_next(top, prev, next);
 	}
 }