static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
{
	if (refcount_dec_and_test(&ctx->refcount)) {
		VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
		VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
		VM_BUG_ON(spin_is_locked(&ctx->event_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->event_wqh));
		VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
		mmdrop(ctx->mm);
		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
	}
}

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.