linux-kernel - Re: signal: break out of wait loops on kthread

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Date:   Wed, 19 Oct 2022 10:00:23 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org,
        "Intel-gfx@...ts.freedesktop.org" <Intel-gfx@...ts.freedesktop.org>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>
Subject: Re: signal: break out of wait loops on kthread_stop()

On Wed, Oct 19, 2022 at 7:31 AM Tvrtko Ursulin
<tvrtko.ursulin@...ux.intel.com> wrote:
>
>
> Hi,
>
> A question regarding a7c01fa93aeb ("signal: break out of wait loops on
> kthread_stop()") if I may.
>
> We have a bunch code in i915, possibly limited to self tests (ie debug
> builds) but still important for our flows, which spawn kernel threads
> and exercises parts of the driver.
>
> Problem we are hitting with this patch is that code did not really need
> to be signal aware until now. Well to say that more accurately - we were
> able to test the code which is normally executed from userspace, so is
> signal aware, but not worry about -ERESTARTSYS or -EINTR within the test
> cases itself.
>
> For example threads which exercise an internal API for a while until the
> parent calls kthread_stop. Now those tests can hit unexpected errors.
>
> Question is how to best approach working around this change. It is of
> course technically possible to rework our code in more than one way,
> although with some cost and impact already felt due reduced pass rates
> in our automated test suites.
>
> Maybe an opt out kthread flag from this new behavior? Would that be
> acceptable as a quick fix? Or any other comments?

You can opt out by running `clear_tsk_thread_flag(current,
TIF_NOTIFY_SIGNAL);` at the top of your kthread. But you should really
fix your code instead. Were I your reviewer, I wouldn't merge code
that took the lazy path like that. However, that should work, if you
do opt for the quick fix.

Jason