mm/swap_slots.c: fix race conditions in swap_slots cache init

Memory allocations can happen before the swap_slots cache initialization
is completed during cpu bring up. If we are low on memory, we could
call get_swap_page() and access swap_slots_cache before it is fully
initialized.

Add a check in get_swap_page() for initialized swap_slots_cache to
prevent this condition. Similar check already exists in free_swap_slot.
Also annotate the checks to indicate the likely condition.

We also added a memory barrier to make sure that the locks
initialization are done before the assignment of cache->slots and
cache->slots_ret pointers. This ensures the assumption that it is safe
to acquire the slots cache locks and use the slots cache when the
corresponding cache->slots or cache->slots_ret pointers are non null.

[[e-mail]: tidy up comment]
[[e-mail]: fix spello in comment]
Link: http://lkml.kernel.org/r/65[e-mail]
Signed-off-by: Tim Chen <[e-mail]>
Reported-by: Wenwei Tao <[e-mail]>
Acked-by: Ying Huang <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Hillf Danton <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/swap_slots.c

···
 	cache->nr = 0;
 	cache->cur = 0;
 	cache->n_ret = 0;
+	/*
+	 * We initialized alloc_lock and free_lock earlier.  We use
+	 * !cache->slots or !cache->slots_ret to know if it is safe to acquire
+	 * the corresponding lock and use the cache.  Memory barrier below
+	 * ensures the assumption.
+	 */
+	mb();
 	cache->slots = slots;
 	slots = NULL;
 	cache->slots_ret = slots_ret;
···
 	struct swap_slots_cache *cache;
 
 	cache = raw_cpu_ptr(&swp_slots);
-	if (use_swap_slot_cache && cache->slots_ret) {
+	if (likely(use_swap_slot_cache && cache->slots_ret)) {
 		spin_lock_irq(&cache->free_lock);
 		/* Swap slots cache may be deactivated before acquiring lock */
 		if (!use_swap_slot_cache || !cache->slots_ret) {
···
 	 */
 	cache = raw_cpu_ptr(&swp_slots);
 
-	if (check_cache_active()) {
+	if (likely(check_cache_active() && cache->slots)) {
 		mutex_lock(&cache->alloc_lock);
 		if (cache->slots) {
 repeat: