z3fold: fix potential race in z3fold_reclaim_page

It is possible that on a (partially) unsuccessful page reclaim,
kref_put() called in z3fold_reclaim_page() does not yield page release,
but the page is released shortly afterwards by another thread.  Then
z3fold_reclaim_page() would try to list_add() that (released) page again
which is obviously a bug.

To avoid that, spin_lock() has to be taken earlier, before the
kref_put() call mentioned earlier.

Link: http://lkml.kernel.org/r/20170913162937.bfff21c7d12b12a5f47639fd@gmail.com
Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: <Oleksiy.Avramchenko@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -875,16 +875,18 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 				goto next;
 		}
 next:
+		spin_lock(&pool->lock);
 		if (test_bit(PAGE_HEADLESS, &page->private)) {
 			if (ret == 0) {
+				spin_unlock(&pool->lock);
 				free_z3fold_page(page);
 				return 0;
 			}
 		} else if (kref_put(&zhdr->refcount, release_z3fold_page)) {
 			atomic64_dec(&pool->pages_nr);
+			spin_unlock(&pool->lock);
 			return 0;
 		}
-		spin_lock(&pool->lock);
 
 		/*
 		 * Add to the beginning of LRU.