static int try_to_merge_one_page(struct vm_area_struct *vma,
				 struct page *page, struct page *kpage)
{
	pte_t orig_pte = __pte(0);
	int err = -EFAULT;

	if (page == kpage)			/* ksm page forked */
		return 0;

	if (!(vma->vm_flags & VM_MERGEABLE))
		goto out;
	if (PageTransCompound(page) && page_trans_compound_anon_split(page))
		goto out;
	BUG_ON(PageTransCompound(page));
	if (!PageAnon(page))
		goto out;

	/*
	 * We need the page lock to read a stable PageSwapCache in
	 * write_protect_page().  We use trylock_page() instead of
	 * lock_page() because we don't want to wait here - we
	 * prefer to continue scanning and merging different pages,
	 * then come back to this page when it is unlocked.
	 */
	if (!trylock_page(page))
		goto out;
	/*
	 * If this anonymous page is mapped only here, its pte may need
	 * to be write-protected.  If it's mapped elsewhere, all of its
	 * ptes are necessarily already write-protected.  But in either
	 * case, we need to lock and check page_count is not raised.
	 */
	if (write_protect_page(vma, page, &orig_pte) == 0) {
		if (!kpage) {
			/*
			 * While we hold page lock, upgrade page from
			 * PageAnon+anon_vma to PageKsm+NULL stable_node:
			 * stable_tree_insert() will update stable_node.
			 */
			set_page_stable_node(page, NULL);
			mark_page_accessed(page);
			err = 0;
		} else if (pages_identical(page, kpage))
			err = replace_page(vma, page, kpage, orig_pte);
	}

	if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
		munlock_vma_page(page);
		if (!PageMlocked(kpage)) {
			unlock_page(page);
			lock_page(kpage);
			mlock_vma_page(kpage);
			page = kpage;		/* for final unlock */
		}
	}

	unlock_page(page);
out:
	return err;
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.