linux-kernel - Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

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Date:   Mon, 23 Sep 2019 18:41:00 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

On 2019-09-17 11:12:48 [-0500], Scott Wood wrote:
> > rcu_read_lock() does:
> > >         __rcu_read_lock();
> > >         __acquire(RCU);
> > >         rcu_lock_acquire(&rcu_lock_map);
> > >         RCU_LOCKDEP_WARN(!rcu_is_watching(),
> > >                          "rcu_read_lock() used illegally while idle");
> > 
> > __acquire() is removed removed by cpp.
> > That RCU_LOCKDEP_WARN is doing the same thing as above and redundant.
> > Am I right to assume that you consider
> > 	rcu_lock_acquire(&rcu_bh_lock_map);
> > 
> > redundant because the only user of that is also checking for
> > rcu_lock_map?
> Yes.

I'm going to drop that hunk. It makes the patch smaller and result more
obvious. If the additional lock annotation (rcu_bh_lock_map) is too much
then we can still remove it later entirely (for RT) but for now I would
like to keep the changes simple and small.

> -Scott

Sebastian