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Message-ID: <20190712151051.GB235410@google.com>
Date: Fri, 12 Jul 2019 11:10:51 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Alexey Kuznetsov <kuznet@....inr.ac.ru>,
Bjorn Helgaas <bhelgaas@...gle.com>, Borislav Petkov <bp@...en8.de>,
c0d1n61at3@...il.com, "David S. Miller" <davem@...emloft.net>,
edumazet@...gle.com,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Josh Triplett <josh@...htriplett.org>, keescook@...omium.org,
kernel-hardening@...ts.openwall.com,
Lai Jiangshan <jiangshanlai@...il.com>, Len Brown <lenb@...nel.org>,
linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
linux-pm@...r.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, neilb@...e.com,
netdev@...r.kernel.org, oleg@...hat.com,
"Paul E. McKenney" <paulmck@...ux.ibm.com>,
Pavel Machek <pavel@....cz>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Rasmus Villemoes <rasmus.villemoes@...vas.dk>, rcu@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, will@...nel.org,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH v1 1/6] rcu: Add support for consolidated-RCU reader
checking
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);
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