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

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Date:   Tue, 17 Sep 2019 11:12:48 -0500
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Tue, 2019-09-17 at 16:42 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-09-17 09:06:28 [-0500], Scott Wood wrote:
> > Sorry, I missed that you were asking about rcu_read_lock_bh() as
> > well.  I
> > did remove the change to rcu_read_lock_bh_held().
> 
> Sorry for not being clear here.
> 
> > With this patch, local_bh_disable() calls rcu_read_lock() on RT which
> > handles this debug stuff.  Doing it twice shouldn't be explicitly
> > harmful,
> > but it's redundant, and debug kernels are slow enough as is.
> 
> 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.

-Scott