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Date:   Fri, 5 May 2023 15:32:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Wander Lairson Costa <wander@redhat.com>
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>,
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        Dietmar Eggemann <dietmar.eggemann@arm.com>,
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        Paul McKenney <paulmck@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe()
 function
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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? Should 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() == 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.