linux-kernel - Re: [PATCH 2/4] rcu/tasks: Handle new PF

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Message-ID: <20231027192026.GG26550@noisy.programming.kicks-ass.net>
Date:   Fri, 27 Oct 2023 21:20:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
        Zqiang <qiang.zhang1211@...il.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH 2/4] rcu/tasks: Handle new PF_IDLE semantics

On Fri, Oct 27, 2023 at 04:40:48PM +0200, Frederic Weisbecker wrote:

> +	/* Has the task been seen voluntarily sleeping? */
> +	if (!READ_ONCE(t->on_rq))
> +		return false;

> -	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {

AFAICT this ->on_rq usage is outside of scheduler locks and that
READ_ONCE isn't going to help much.

Obviously a pre-existing issue, and I suppose all it cares about is
seeing a 0 or not, irrespective of the races, but urgh..