linux-kernel - Re: [PATCH] sched/fair: Fix call walk_tg_tree_from() without hold rcu

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Message-ID: <20200421135258.GS20730@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Apr 2020 15:52:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mingo@...nel.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, paulmck@...nel.org, joel@...lfernandes.org
Subject: Re: [PATCH] sched/fair: Fix call walk_tg_tree_from() without hold
 rcu_lock

On Mon, Apr 06, 2020 at 08:10:08PM +0800, Muchun Song wrote:
> The walk_tg_tree_from() caller must hold rcu_lock,

Not quite; with the RCU unification done 'recently' having preemption
disabled is sufficient. AFAICT preemption is disabled.

In fact; and I mentioned this to someone the other day, perhaps Joel; we
can go and delete a whole bunch of rcu_read_lock() from the scheduler --
basically undo all the work we did after RCU was split many years ago.