static int userfaultfd_release(struct inode *inode, struct file *file)
{
	struct userfaultfd_ctx *ctx = file->private_data;
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma, *prev;
	/* len == 0 means wake all */
	struct userfaultfd_wake_range range = { .len = 0, };
	unsigned long new_flags;

	WRITE_ONCE(ctx->released, true);

	if (!mmget_not_zero(mm))
		goto wakeup;

	/*
	 * Flush page faults out of all CPUs. NOTE: all page faults
	 * must be retried without returning VM_FAULT_SIGBUS if
	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
	 * changes while handle_userfault released the mmap_sem. So
	 * it's critical that released is set to true (above), before
 	 * taking the mmap_sem for writing.
 	 */
 	down_write(&mm->mmap_sem);
	if (!mmget_still_valid(mm))
		goto skip_mm;
 	prev = NULL;
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		cond_resched();
		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
		       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
			prev = vma;
			continue;
		}
		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
				 new_flags, vma->anon_vma,
				 vma->vm_file, vma->vm_pgoff,
				 vma_policy(vma),
				 NULL_VM_UFFD_CTX);
		if (prev)
			vma = prev;
		else
			prev = vma;
 		vma->vm_flags = new_flags;
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 	}
skip_mm:
 	up_write(&mm->mmap_sem);
 	mmput(mm);
 wakeup:
	/*
	 * After no new page faults can wait on this fault_*wqh, flush
	 * the last page faults that may have been already waiting on
	 * the fault_*wqh.
	 */
	spin_lock(&ctx->fault_pending_wqh.lock);
	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
	__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
	spin_unlock(&ctx->fault_pending_wqh.lock);

	/* Flush pending events that may still wait on event_wqh */
	wake_up_all(&ctx->event_wqh);

	wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
	userfaultfd_ctx_put(ctx);
	return 0;
}

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.