linux-kernel - Re: [PATCH] sched/deadline: fix inactive_task_timer splat with CONFIG_PREEMPT

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Message-ID: <xhsmh7cxjitov.mognet@vschneid.remote.csb>
Date:   Wed, 18 Jan 2023 18:28:00 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     paulmck@...nel.org
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "open list:SCHEDULER" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/deadline: fix inactive_task_timer splat with
 CONFIG_PREEMPT_RT

On 18/01/23 10:11, Paul E. McKenney wrote:
> On Wed, Jan 18, 2023 at 03:57:38PM +0000, Valentin Schneider wrote:
>> > Given that this problem occurred in PREEMPT_RT, I am assuming that the
>> > appropriate definition of "atomic context" is "cannot call schedule()".
>> > And you are in fact not permitted to call schedule() from a bh-disabled
>> > region.
>> >
>> > This also means that you cannot acquire a non-raw spinlock in a
>> > bh-disabled region of code in a PREEMPT_RT kernel, because doing
>> > so can invoke schedule.
>>
>> But per the PREEMPT_RT lock "replacement", non-raw spinlocks end up
>> invoking schedule_rtlock(), which should be safe vs BH disabled
>> (local_lock() + rcu_read_lock()):
>>
>>   6991436c2b5d ("sched/core: Provide a scheduling point for RT locks")
>>
>> Unless I'm missing something else?
>
> No, you miss nothing.  Apologies for my confusion!
>
> (I could have sworn that someone else corrected me on this earlier,
> but I don't see it right off hand.)
>
>                                                       Thanx, Paul

Heh, I had a smidge of doubt myself, but since we've cleared this up:

Reviewed-by: Valentin Schneider <vschneid@...hat.com>