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Subject: Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs
From:   Scott Wood <swood@redhat.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Clark Williams <williams@redhat.com>
Date:   Mon, 26 Aug 2019 18:21:46 -0500
In-Reply-To: <20190826155943.zvghokdn3iu2sipx@linutronix.de>
References: <20190821231906.4224-1-swood@redhat.com>
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         <d65032399f66ec85731fdcf4f8c6c7af74687fb2.camel@redhat.com>
         <20190823161740.xhntflxs3vlf3xnu@linutronix.de>
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On Mon, 2019-08-26 at 17:59 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-08-23 14:46:39 [-0500], Scott Wood wrote:
> > > > Before consolidation, RT mapped rcu_read_lock_bh_held() to
> > > > rcu_read_lock_bh() and called rcu_read_lock() from
> > > > rcu_read_lock_bh().  This
> > > > somehow got lost when rebasing on top of 5.0.
> > > 
> > > so now rcu_read_lock_bh_held() is untouched and in_softirq() reports
> > > 1.
> > > So the problem is that we never hold RCU but report 1 like we do?
> > 
> > Yes.
> 
> I understand the part where "rcu_read_lock() becomes part of
> local_bh_disable()". But why do you modify rcu_read_lock_bh_held() and
> rcu_read_lock_bh()? Couldn't they remain as-is?

Yes, it looks like they can.  I recall having problems with
rcu_read_lock_bh_held() in an earlier version which is what prompted the
change, but looking back I don't see what the problem was.

-Scott