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To:     Brendan Higgins <brendanhiggins@google.com>
From:   Stephen Boyd <sboyd@kernel.org>
Subject: Re: [PATCH v5 01/18] kunit: test: add KUnit test runner core
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Date:   Thu, 27 Jun 2019 11:16:35 -0700
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Quoting Brendan Higgins (2019-06-26 16:00:40)
> On Tue, Jun 25, 2019 at 8:41 PM Stephen Boyd <sboyd@kernel.org> wrote:
>=20
> > scenario like below, but where it is a problem. There could be three
> > CPUs, or even one CPU and three threads if you want to describe the
> > extra thread scenario.
> >
> > Here's my scenario where it isn't needed:
> >
> >     CPU0                                      CPU1
> >     ----                                      ----
> >     kunit_run_test(&test)
> >                                               test_case_func()
> >                                                 ....
> >                                               [mock hardirq]
> >                                                 kunit_set_success(&test)
> >                                               [hardirq ends]
> >                                                 ...
> >                                                 complete(&test_done)
> >       wait_for_completion(&test_done)
> >       kunit_get_success(&test)
> >
> > We don't need to care about having locking here because success or
> > failure only happens in one place and it's synchronized with the
> > completion.
>=20
> Here is the scenario I am concerned about:
>=20
> CPU0                      CPU1                       CPU2
> ----                      ----                       ----
> kunit_run_test(&test)
>                           test_case_func()
>                             ....
>                             schedule_work(foo_func)
>                           [mock hardirq]             foo_func()
>                             ...                        ...
>                             kunit_set_success(false)   kunit_set_success(=
false)
>                           [hardirq ends]               ...
>                             ...
>                             complete(&test_done)
>   wait_for_completion(...)
>   kunit_get_success(&test)
>=20
> In my scenario, since both CPU1 and CPU2 update the success status of
> the test simultaneously, even though they are setting it to the same
> value. If my understanding is correct, this could result in a
> write-tear on some architectures in some circumstances. I suppose we
> could just make it an atomic boolean, but I figured locking is also
> fine, and generally preferred.

This is what we have WRITE_ONCE() and READ_ONCE() for. Maybe you could
just use that in the getter and setters and remove the lock if it isn't
used for anything else.

It may also be a good idea to have a kunit_fail_test() API that fails
the test passed in with a WRITE_ONCE(false). Otherwise, the test is
assumed successful and it isn't even possible for a test to change the
state from failure to success due to a logical error because the API
isn't available. Then we don't really need to have a generic
kunit_set_success() function at all. We could have a kunit_test_failed()
function too that replaces the kunit_get_success() function. That would
read better in an if condition.

>=20
> Also, to be clear, I am onboard with dropping then IRQ stuff for now.
> I am fine moving to a mutex for the time being.
>=20

Ok.