linux-kernel - Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic

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Message-ID: <20230424163438.06cc880e@rorschach.local.home>
Date:   Mon, 24 Apr 2023 16:34:38 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        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>,
        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>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v6 2/3] sched/task: Add the
 put_task_struct_atomic_safe() function

On Mon, 24 Apr 2023 11:52:52 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > softirqs execute in thread context in PREEMPT_RT. We are good here.  
> 
> So the sleeping lock is a spinlock rather than (say) a mutex?

local_bh_disable() on RT basically turns into:

	local_lock(&softirq_ctrl.lock);
	rcu_read_lock();

Which grabs a per CPU mutex that is taken by softirqs, and also calls
rcu_read_lock(). This allows bottom halves to still run as threads but
maintain the same synchronization as they do on mainline.

-- Steve