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From:   Wander Lairson Costa <wander@redhat.com>
Date:   Mon, 8 May 2023 09:28:16 -0300
Message-ID: <CAAq0SUmf_q5UFMKrBb+JtqUF3cFchSZhiiRM4YbVmYO_dyDRGg@mail.gmail.com>
Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
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On Fri, May 5, 2023 at 10:33=E2=80=AFAM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Thu, May 04, 2023 at 03:21:11PM -0300, Wander Lairson Costa wrote:
> > On Thu, May 04, 2023 at 05:24:24PM +0200, Peter Zijlstra wrote:
> > > On Thu, May 04, 2023 at 11:55:15AM -0300, Wander Lairson Costa wrote:
> > >
> > > > > Then I'm thinking something trivial like so:
> > > > >
> > > > > static inline void put_task_struct(struct task_struct *t)
> > > > > {
> > > > >         if (!refcount_dec_and_test(&t->usage))
> > > > >                 return;
> > > > >
> > > > >         if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > > > >                 call_rcu(&t->rcu, __put_task_struct_rcu);
> > > > >
> > > > >         __put_task_struct(t);
> > > > > }
> > > > >
> > > >
> > > > That's what v5 [1] does. What would be the path in this case? Shoul=
d I
> > > > resend it as v8?
> > >
> > > It's almost what v5 does. v5 also has a !in_task() thing. v5 also
> > > violates codingstyle :-)
> >
> > IIRC, the in_task() is there because preemptible() doesn't check if it
> > is running in interrupt context.
>
> #define preemptible()   (preempt_count() =3D=3D 0 && !irqs_disabled())
>
> When in interrupt context preempt_count() will have a non-zero value in
> HARDIRQ_MASK and IRQs must be disabled, so preemptible() evaluates to
> (false && false), last time I checked that ends up being false.
>

When I first tested, I started with in_irq(), then I saw some warnings
and added preemptible().
Never tested with preemptible() alone. Thanks, I will change that and test =
it.