void ksm_migrate_page(struct page *newpage, struct page *oldpage)
{
	struct stable_node *stable_node;

	VM_BUG_ON(!PageLocked(oldpage));
	VM_BUG_ON(!PageLocked(newpage));
	VM_BUG_ON(newpage->mapping != oldpage->mapping);

	stable_node = page_stable_node(newpage);
	if (stable_node) {
		VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage));
		stable_node->kpfn = page_to_pfn(newpage);
	}
}

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.