linux-kernel - Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs

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Message-ID: <40dd3a7e37ed9b3d03c50221dafc7aa137827ce8.camel@redhat.com>
Date:   Fri, 23 Aug 2019 14:46:39 -0500
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs

On Fri, 2019-08-23 at 18:17 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-08-22 22:23:23 [-0500], Scott Wood wrote:
> > On Thu, 2019-08-22 at 09:39 -0400, Joel Fernandes wrote:
> > > On Wed, Aug 21, 2019 at 04:33:58PM -0700, Paul E. McKenney wrote:
> > > > On Wed, Aug 21, 2019 at 06:19:04PM -0500, Scott Wood wrote:
> > > > > Signed-off-by: Scott Wood <swood@...hat.com>
> > > > > ---
> > > > > Another question is whether non-raw spinlocks are intended to
> > > > > create
> > > > > an
> > > > > RCU read-side critical section due to implicit preempt disable.
> > > > 
> > > > Hmmm...  Did non-raw spinlocks act like rcu_read_lock_sched()
> > > > and rcu_read_unlock_sched() pairs in -rt prior to the RCU flavor
> > > > consolidation?  If not, I don't see why they should do so after that
> > > > consolidation in -rt.
> > > 
> > > May be I am missing something, but I didn't see the connection between
> > > consolidation and this patch. AFAICS, this patch is so that
> > > rcu_read_lock_bh_held() works at all on -rt. Did I badly miss
> > > something?
> > 
> > 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.

-Scott