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

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Message-ID: <20190823015009.GA152050@google.com>
Date:   Thu, 22 Aug 2019 21:50:09 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Scott Wood <swood@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 Thu, Aug 22, 2019 at 08:27:06AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 22, 2019 at 09:39:55AM -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:
> > > > A plain local_bh_disable() is documented as creating an RCU critical
> > > > section, and (at least) rcutorture expects this to be the case.  However,
> > > > in_softirq() doesn't block a grace period on PREEMPT_RT, since RCU checks
> > > > preempt_count() directly.  Even if RCU were changed to check
> > > > in_softirq(), that wouldn't allow blocked BH disablers to be boosted.
> > > > 
> > > > Fix this by calling rcu_read_lock() from local_bh_disable(), and update
> > > > rcu_read_lock_bh_held() accordingly.
> > > 
> > > Cool!  Some questions and comments below.
> > > 
> > > 							Thanx, Paul
> > > 
> > > > 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?
> 
> I was interpreting Scott's question (which would be excluded from the
> git commit log) as relating to a possible follow-on patch.
> 
> The question is "how special can non-raw spinlocks be in -rt?".  From what
> I can see, they have been treated as sleeplocks from an RCU viewpoint,
> so maybe that should continue to be the case.  It does deserve some
> thought because in mainline a non-raw spinlock really would block a
> post-consolidation RCU grace period, even in PREEMPT kernels.
> 
> But then again, you cannot preempt a non-raw spinlock in mainline but
> you can in -rt, so extending that exception to RCU is not unreasonable.
> 
> Either way, we do need to make a definite decision and document it.
> If I were forced to make a decision right now, I would follow the old
> behavior, so that only raw spinlocks were guaranteed to block RCU grace
> periods.  But I am not being forced, so let's actually discuss and make
> a conscious decision.  ;-)

I think non-raw spinlocks on -rt should at least do rcu_read_lock() so that
any driver or kernel code that depends on this behavior and works on non-rt
also works on -rt. It also removes the chance a kernel developer may miss
documentation and accidentally forget that their code may break on -rt. I am
curious to see how much this design pattern appears in the kernel
(spin_lock'ed section "intended" as an RCU-reader by code sequences).

Logically speaking, to me anything that disables preemption on non-RT should
do rcu_read_lock() on -rt so that from RCU's perspective, things are working.
But I wonder where we would draw the line and if the bar is to need actual
examples of usage patterns to make a decision..

Any thoughts?

thanks,

 - Joel