mm: slub: be more careful about the double cmpxchg of freelist

This is just a cleanup addition to Jann's fix to properly update the
transaction ID for the slub slowpath in commit fd4d9c7d ("mm: slub:
add missing TID bump..").

The transaction ID is what protects us against any concurrent accesses,
but we should really also make sure to make the 'freelist' comparison
itself always use the same freelist value that we then used as the new
next free pointer.

Jann points out that if we do all of this carefully, we could skip the
transaction ID update for all the paths that only remove entries from
the lists, and only update the TID when adding entries (to avoid the ABA
issue with cmpxchg and list handling re-adding a previously seen value).

But this patch just does the "make sure to cmpxchg the same value we
used" rather than then try to be clever.
Acked-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -2997,11 +2997,13 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
 	barrier();
 
 	if (likely(page == c->page)) {
-		set_freepointer(s, tail_obj, c->freelist);
+		void **freelist = READ_ONCE(c->freelist);
+
+		set_freepointer(s, tail_obj, freelist);
 
 		if (unlikely(!this_cpu_cmpxchg_double(
 				s->cpu_slab->freelist, s->cpu_slab->tid,
-				c->freelist, tid,
+				freelist, tid,
 				head, next_tid(tid)))) {
 
 			note_cmpxchg_failure("slab_free", s, tid);