arch_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
			  unsigned long len, unsigned long pgoff,
			  unsigned long flags)
{
	struct vm_area_struct *vma, *prev;
	struct mm_struct *mm = current->mm;
	struct vm_unmapped_area_info info;
	const unsigned long mmap_end = arch_get_mmap_end(addr);

	/* requested length too big for entire address space */
	if (len > mmap_end - mmap_min_addr)
		return -ENOMEM;

	if (flags & MAP_FIXED)
		return addr;

	/* requesting a specific address */
	if (addr) {
		addr = PAGE_ALIGN(addr);
		vma = find_vma_prev(mm, addr, &prev);
		if (mmap_end - len >= addr && addr >= mmap_min_addr &&
				(!vma || addr + len <= vm_start_gap(vma)) &&
				(!prev || addr >= vm_end_gap(prev)))
			return addr;
	}

	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
	info.length = len;
	info.low_limit = max(PAGE_SIZE, mmap_min_addr);
	info.high_limit = arch_get_mmap_base(addr, mm->mmap_base);
	info.align_mask = 0;
	addr = vm_unmapped_area(&info);

	/*
	 * A failed mmap() very likely causes application failure,
	 * so fall back to the bottom-up function here. This scenario
	 * can happen with large stack limits and large mmap()
	 * allocations.
	 */
	if (offset_in_page(addr)) {
		VM_BUG_ON(addr != -ENOMEM);
		info.flags = 0;
		info.low_limit = TASK_UNMAPPED_BASE;
		info.high_limit = mmap_end;
		addr = vm_unmapped_area(&info);
	}

	return addr;
}

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.