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References: <20200911081745.214686199@infradead.org> <20200911082536.470013100@infradead.org> <jhjsgbo5wzl.mognet@arm.com> <20200911122547.GI1362448@hirez.programming.kicks-ass.net>
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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     peterz@infradead.org
Cc:     mingo@kernel.org, vincent.guittot@linaro.org, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bristot@redhat.com, swood@redhat.com
Subject: Re: [PATCH 1/2] sched: Fix balance_callback()
In-reply-to: <20200911122547.GI1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Sep 2020 14:27:30 +0100
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On 11/09/20 13:25, peterz@infradead.org wrote:
> On Fri, Sep 11, 2020 at 01:17:02PM +0100, Valentin Schneider wrote:
>> So that can be say __schedule() tail racing with some setprio; what's the
>> worst that can (currently) happen here? Something like say two consecutive
>> enqueuing of push_rt_tasks() to the callback list?
>
> Yeah, but that isn't in fact the case I worry most about.
>
> What can happen (and what I've spotted once before) is that someone
> attempts to enqueue a balance_callback from a rq->lock region that
> doesn't handle the calls.
>
> Currently that 'works', that is, it will get ran _eventually_. But
> ideally we'd want that to not work and issue a WARN. We want the
> callbacks to be timely.
>
> So basically all of these machinations we in order to add the WARN :-)

Makes sense, thanks!