Re: [RFC PATCH 09/12] rcu_txn: Test a list writer against concurrent readers

[Puranjay Mohan]

On Fri, Oct 16, 2026 at 09:33:16AM -0400, Mathieu Desnoyers wrote:
[...]

> +			/*
> +			 * Invariant (2).  Skipped for the first step, whose
> +			 * predecessor is the head.
> +			 */
> +			if (insert_only && prev_key) {
> +				struct rcu_txn_list_node *back =
> +						rcu_txn_list_prev_rcu(p);
> +
> +				r->cross_checks++;
> +				if (back != &head->node &&
> +				    container_of(back, struct cn,
> +						 node)->key < prev_key)
> +					r->cross_violations++;
> +			}

In an insert-only phase, once a reader has stepped from A to p, A stays
before p, so p's prev can only be A or a node inserted between them,
never the head.  The back != &head->node exemption can therefore only
hide failures, and it hides exactly the tear this check is for when the
insertion is at the front: head->next == E, E->next == F, then
F->prev == head.  prev_key already skips the first step, so I'd count
back == &head->node as a violation.

[...]

> +	for (i = 0; i < CONC_READERS; i++) {
> +		readers[i].conc = c;
> +		readers[i].task = kthread_run(conc_reader_fn, &readers[i],
> +					      "rcu_txn_reader/%d", i);
> +		if (IS_ERR(readers[i].task)) {
> +			readers[i].task = NULL;
> +			ok = false;
> +		}
> +	}

If the writer oopses, KUnit fails the test and frees c and the readers
array while these threads are still walking the list.  Registering the
stop with kunit_add_action_or_reset(), and calling
kunit_release_action() where they're stopped now, would avoid the
use-after-free.

[...]

> +	for (key = 1; key <= CONC_KEY_MAX; key++)
> +		if (c->live[key])
> +			conc_del(c, key);

A failed conc_del() here leaks the node without anyone noticing.  The
readers are already stopped at this point, so the leftovers could just
be kfree()d.

Thanks,
Puranjay