static long vma_compute_subtree_gap(struct vm_area_struct *vma)
{
	unsigned long max, prev_end, subtree_gap;

	/*
	 * Note: in the rare case of a VM_GROWSDOWN above a VM_GROWSUP, we
	 * allow two stack_guard_gaps between them here, and when choosing
	 * an unmapped area; whereas when expanding we only require one.
	 * That's a little inconsistent, but keeps the code here simpler.
	 */
	max = vm_start_gap(vma);
	if (vma->vm_prev) {
		prev_end = vm_end_gap(vma->vm_prev);
		if (max > prev_end)
			max -= prev_end;
		else
			max = 0;
	}
	if (vma->vm_rb.rb_left) {
		subtree_gap = rb_entry(vma->vm_rb.rb_left,
				struct vm_area_struct, vm_rb)->rb_subtree_gap;
		if (subtree_gap > max)
			max = subtree_gap;
	}
	if (vma->vm_rb.rb_right) {
		subtree_gap = rb_entry(vma->vm_rb.rb_right,
				struct vm_area_struct, vm_rb)->rb_subtree_gap;
		if (subtree_gap > max)
			max = subtree_gap;
	}
	return max;
}

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.