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

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Date:   Fri, 5 May 2023 10:26:02 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        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>,
        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 Fri, 5 May 2023 15:32:35 +0200
Peter Zijlstra <peterz@...radead.org> 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? 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.

Interesting, I can't find v5 anywhere in my mail folders (but I have
v4 and v6!). Anyway, from just the context of this email, and seeing
IS_ENABLED(CONFIG_PREEMPT_RT), I'm guessing that in_task() returns false if
it's running in a interrupt thread, where preemtible() does not.

-- Steve