Re: [PATCH net-next 06/10] dpll: add KUnit tests for core locking

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- [Medium] In drivers/dpll/dpll_core_kunit.c, some tests set a blocking
  flag before dt_req_start() has registered dt_req_release().

--

dpll: add KUnit tests for core locking

This adds KUnit tests for DPLL core locking. They use a mock driver with
several domains to check domain sharing, which locks callbacks hold,
independence and serialization of domains, atomicity of multi-attribute
pin sets, and notification ordering. The netlink pre/post doit and set
handlers are exported for KUnit so the tests can call them directly.

> diff --git a/drivers/dpll/dpll_core_kunit.c b/drivers/dpll/dpll_core_kunit.c
> new file mode 100644
> index 0000000000000..a58800eda5ff5
> --- /dev/null
> +++ b/drivers/dpll/dpll_core_kunit.c
> @@ -0,0 +1,1471 @@

[ ... ]

> +static void dt_block(struct dt_dom *dom, bool *block)
> +{
> +	complete(&dom->entered);
> +	wait_event(dom->ctx->wq, !READ_ONCE(*block));
> +}

[ ... ]

> +static struct dt_req *dt_req_start(struct kunit *test, int (*fn)(void *arg),
> +				   void *arg)
> +{
> +	struct task_struct *task;
> +	struct dt_req *req;
> +
> +	req = kunit_kzalloc(test, sizeof(*req), GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_NULL(test, req);
> +
> +	req->ctx = test->priv;
> +	req->fn = fn;
> +	req->arg = arg;
> +	init_completion(&req->done);
> +
> +	task = kthread_run(dt_req_thread, req, "dpll-test");
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, task);
> +	KUNIT_ASSERT_EQ(test, kunit_add_action_or_reset(test, dt_req_release,
> +							req), 0);
> +
> +	return req;
> +}

[ ... ]

> +/* Blocks a request of @dom in a callback, which holds the domain lock */
> +static struct dt_req *dt_block_in(struct kunit *test, struct dt_dom *dom,
> +				  int (*fn)(void *arg), void *arg)
> +{
> +	struct dt_req *req;
> +
> +	WRITE_ONCE(dom->block, true);
> +	req = dt_req_start(test, fn, arg);

[Severity: Medium]
Can the teardown hang here if dt_req_start() fails one of its assertions?

dom->block is set before dt_req_start() registers dt_req_release(), and
dt_req_release() is the only cleanup action that calls dt_unblock(). If
kunit_kzalloc() or kthread_run() in dt_req_start() fails, the
KUNIT_ASSERT_NOT_NULL() or KUNIT_ASSERT_NOT_ERR_OR_NULL() check aborts the
test and the flag stays set.

The cleanup actions that were already registered then run, for example:

dt_reg_release()
  dpll_pin_unregister()
    down_write(&dpll_lock)
    __dpll_pin_unregister()
      dpll_pin_delete_ntf()
        mutex_lock(&pin->domain->lock)
        dpll_pin_event_send()
          dpll_cmd_pin_get_one()
            pin get callbacks
              dt_pin_enter()->dt_enter()->dt_block()

dt_block() then waits with:

    wait_event(dom->ctx->wq, !READ_ONCE(*block));

Nothing is left that can clear the flag. The KUnit cleanup thread would
sleep uninterruptibly while holding dpll_lock for write and a domain
mutex. Every other DPLL user in the system would then block behind it.

dt_test_stuck_foreign_device() has the same problem:

    WRITE_ONCE(ctx->f0.block, true);
    blocked = dt_req_start(test, dt_device_ntf, &ctx->f0);

There, dt_dev_release()->dpll_device_unregister()->dpll_device_delete_ntf()
->dpll_device_get_one()->dt_mode_get()->dt_dev_enter() would reach
dt_block() on f0.block.

One option is to register a cleanup action that calls dt_unblock() from
dt_init(), before any flag can be set. Another is to set the flags only
after dt_req_release() has been registered. Would either of these work?

Setting block_prio_set early looks harmless, since no teardown path calls
prio_set.

> +	KUNIT_ASSERT_TRUE(test, wait_for_completion_timeout(&dom->entered,
> +							    DT_TIMEOUT));
> +
> +	return req;
> +}

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261106121005.1327603-1-ivecera%40redhat.com