static bool get_desc(struct desc_struct *out, unsigned short sel)
{
	struct desc_ptr gdt_desc = {0, 0};
	unsigned long desc_base;

#ifdef CONFIG_MODIFY_LDT_SYSCALL
	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT) {
		bool success = false;
		struct ldt_struct *ldt;

		/* Bits [15:3] contain the index of the desired entry. */
		sel >>= 3;

		mutex_lock(&current->active_mm->context.lock);
		ldt = current->active_mm->context.ldt;
		if (ldt && sel < ldt->nr_entries) {
			*out = ldt->entries[sel];
			success = true;
		}

		mutex_unlock(&current->active_mm->context.lock);

		return success;
	}
#endif
	native_store_gdt(&gdt_desc);

	/*
	 * Segment descriptors have a size of 8 bytes. Thus, the index is
	 * multiplied by 8 to obtain the memory offset of the desired descriptor
	 * from the base of the GDT. As bits [15:3] of the segment selector
	 * contain the index, it can be regarded as multiplied by 8 already.
	 * All that remains is to clear bits [2:0].
	 */
	desc_base = sel & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK);

	if (desc_base > gdt_desc.size)
		return false;

	*out = *(struct desc_struct *)(gdt_desc.address + desc_base);
	return true;
}
The program contains a code sequence that can run concurrently with other code, and the code sequence 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 that is operating concurrently.