Re: [PATCH v1 1/6] rcu: Add support for consolidated-RCU reader checking

From: Joel Fernandes <joel@joelfernandes.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Borislav Petkov <bp@alien8.de>,
	c0d1n61at3@gmail.com, "David S. Miller" <davem@davemloft.net>,
	edumazet@google.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Josh Triplett <josh@joshtriplett.org>,
	keescook@chromium.org, kernel-hardening@lists.openwall.com,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Len Brown <lenb@kernel.org>,
	linux-acpi@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-pm@vger.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	neilb@suse.com, netdev@vger.kernel.org, oleg@redhat.com,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	Pavel Machek <pavel@ucw.cz>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
	rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
	Tejun Heo <tj@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
	will@kernel.org,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>
Subject: Re: [PATCH v1 1/6] rcu: Add support for consolidated-RCU reader checking
Date: Fri, 12 Jul 2019 11:10:51 -0400	[thread overview]
Message-ID: <20190712151051.GB235410@google.com> (raw)
In-Reply-To: <20190712111125.GT3402@hirez.programming.kicks-ass.net>

On Fri, Jul 12, 2019 at 01:11:25PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 11, 2019 at 07:43:56PM -0400, Joel Fernandes (Google) wrote:
> > +int rcu_read_lock_any_held(void)
> > +{
> > +	int lockdep_opinion = 0;
> > +
> > +	if (!debug_lockdep_rcu_enabled())
> > +		return 1;
> > +	if (!rcu_is_watching())
> > +		return 0;
> > +	if (!rcu_lockdep_current_cpu_online())
> > +		return 0;
> > +
> > +	/* Preemptible RCU flavor */
> > +	if (lock_is_held(&rcu_lock_map))
> 
> you forgot debug_locks here.

Actually, it turns out debug_locks checking is not even needed. If
debug_locks == 0, then debug_lockdep_rcu_enabled() returns 0 and we would not
get to this point.

> > +		return 1;
> > +
> > +	/* BH flavor */
> > +	if (in_softirq() || irqs_disabled())
> 
> I'm not sure I'd put irqs_disabled() under BH, also this entire
> condition is superfluous, see below.
> 
> > +		return 1;
> > +
> > +	/* Sched flavor */
> > +	if (debug_locks)
> > +		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> > +	return lockdep_opinion || !preemptible();
> 
> that !preemptible() turns into:
> 
>   !(preempt_count()==0 && !irqs_disabled())
> 
> which is:
> 
>   preempt_count() != 0 || irqs_disabled()
> 
> and already includes irqs_disabled() and in_softirq().
> 
> > +}
> 
> So maybe something lke:
> 
> 	if (debug_locks && (lock_is_held(&rcu_lock_map) ||
> 			    lock_is_held(&rcu_sched_lock_map)))
> 		return true;

Agreed, I will do it this way (without the debug_locks) like:

---8<-----------------------

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ba861d1716d3..339aebc330db 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -296,27 +296,15 @@ EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
 
 int rcu_read_lock_any_held(void)
 {
-	int lockdep_opinion = 0;
-
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
 	if (!rcu_is_watching())
 		return 0;
 	if (!rcu_lockdep_current_cpu_online())
 		return 0;
-
-	/* Preemptible RCU flavor */
-	if (lock_is_held(&rcu_lock_map))
-		return 1;
-
-	/* BH flavor */
-	if (in_softirq() || irqs_disabled())
-		return 1;
-
-	/* Sched flavor */
-	if (debug_locks)
-		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
-	return lockdep_opinion || !preemptible();
+	if (lock_is_held(&rcu_lock_map) || lock_is_held(&rcu_sched_lock_map))
+		return 1;
+	return !preemptible();
 }
 EXPORT_SYMBOL_GPL(rcu_read_lock_any_held);
 
