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

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Message-ID: <20200421162452.GV20730@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Apr 2020 18:24:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Muchun Song <songmuchun@...edance.com>, 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,
        joel@...lfernandes.org
Subject: Re: [PATCH] sched/fair: Fix call walk_tg_tree_from() without hold
 rcu_lock

On Tue, Apr 21, 2020 at 08:43:12AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 21, 2020 at 03:52:58PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> "If only I knew then what I know now..."
> 
> Then again, I suspect that we all have ample opportunity to use that
> particular old phrase.  ;-)

Quite so; I'm just fearing that rcu-lockdep annotation stuff. IIRC that
doesn't (nor can it, in general) consider the implicit preempt-disable
from locks and such for !PREEMPT builds.