linux-kernel - Re: [PATCH RT 1/4] rcu: Acquire RCU lock when disabling BHs

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Message-Id: <20190620221607.GA26519@linux.ibm.com>
Date:   Thu, 20 Jun 2019 15:16:07 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RT 1/4] rcu: Acquire RCU lock when disabling BHs

On Thu, Jun 20, 2019 at 04:38:47PM -0500, Scott Wood wrote:
> On Thu, 2019-06-20 at 14:20 -0700, Paul E. McKenney wrote:
> > On Thu, Jun 20, 2019 at 04:06:02PM -0500, Scott Wood wrote:
> > > On Thu, 2019-06-20 at 13:53 -0700, Paul E. McKenney wrote:
> > > > And I have to ask...
> > > > 
> > > > What did you do to test this change to kernel/softirq.c?  My past
> > > > attempts
> > > > to do this sort of thing have always run afoul of open-coded BH
> > > > transitions.
> > > 
> > > Mostly rcutorture and loads such as kernel builds, on a debug
> > > kernel.  By
> > > "open-coded BH transition" do you mean directly manipulating the preempt
> > > count?  That would already be broken on RT.
> > 
> > OK, then maybe you guys have already done the needed cleanup work.  Cool!
> 
> Do you remember what code was doing such things?  Grepping for the obvious
> things doesn't turn up anything outside the softirq code, even in the
> earlier non-RT kernels I checked.

It was many years ago, and it is quite possible that I am conflating
irqs with bh or some such.  If it now works, it now works.

> > But don't the additions of rcu_read_lock() and rcu_read_unlock() want
> > to be protected by "!IS_ENABLED(CONFIG_PREEMPT_RT_FULL)" or similar?
> 
> This is already a separate PREEMPT_RT_FULL-specific implementation.

Ah, sorry for the noise, then!

							Thanx, Paul