static void vm_unlock_anon_vma(struct anon_vma *anon_vma)
{
	if (test_bit(0, (unsigned long *) &anon_vma->root->rb_root.rb_root.rb_node)) {
		/*
		 * The LSB of head.next can't change to 0 from under
		 * us because we hold the mm_all_locks_mutex.
		 *
		 * We must however clear the bitflag before unlocking
		 * the vma so the users using the anon_vma->rb_root will
		 * never see our bitflag.
		 *
		 * No need of atomic instructions here, head.next
		 * can't change from under us until we release the
		 * anon_vma->root->rwsem.
		 */
		if (!__test_and_clear_bit(0, (unsigned long *)
					  &anon_vma->root->rb_root.rb_root.rb_node))
			BUG();
		anon_vma_unlock_write(anon_vma);
	}
}

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.