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Date:   Tue, 17 Sep 2019 16:42:52 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Scott Wood <swood@redhat.com>
Cc:     linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Paul E . McKenney" <paulmck@linux.ibm.com>,
        Joel Fernandes <joel@joelfernandes.org>,
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        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Clark Williams <williams@redhat.com>
Subject: Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs
Message-ID: <20190917144252.v34ina4z2ydchoit@linutronix.de>
References: <20190911165729.11178-1-swood@redhat.com>
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 <20190917074456.yj7t3wdwuhn3zcng@linutronix.de>
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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?

> -Scott

Sebastian