linux-kernel - Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic

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Date:   Thu, 4 May 2023 16:38:16 -0300
From:   Wander Lairson Costa <wander@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        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>,
        Valentin Schneider <vschneid@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Brian Cain <bcain@...cinc.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Brauner <brauner@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@...r.kernel.org>, Hu Chunyu <chuhu@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function

On Thu, May 4, 2023 at 4:23 PM Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 05/04, Wander Lairson Costa wrote:
> >
> > On Thu, May 4, 2023 at 12:23 PM Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > Yes, but as Sebastian explained CONFIG_PROVE_RAW_LOCK_NESTING won't like it.
> > >
> > >         https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@linutronix.de/
> > >
> >
> > I think that was my confusion in that thread. My understanding is that
> > CONFIG_PROVE_RAW_LOCK_NESTING will check lock ordering but not
> > context.
>
> Sorry, I don't understand... perhaps I missed something. But iiuc
> the problem is simple.
>
> So, this code
>
>         raw_spin_lock(one);
>         spin_lock(two);
>
> is obviously wrong if CONFIG_PREEMPT_RT.
>
> Without PREEMPT_RT this code is fine because raw_spinlock_t and spinlock_t
> are the same thing. Except they have different lockdep annotations if
> CONFIG_PROVE_RAW_LOCK_NESTING is true, LD_WAIT_SPIN and LD_WAIT_CONFIG.
>
> So if CONFIG_PROVE_RAW_LOCK_NESTING is set, lockdep will complain even
> on the !PREEMPT_RT kernel, iow it checks the nesting as if the code runs
> on with PREEMPT_RT.
>
> Cough... not sure my explanation can help ;) It looks very confusing when
> I read it.
>

Thanks for the explanation. That's my understanding too. The part I
don't get is why this would fail with a call_rcu() inside
put_task_struct().

> Oleg.
>