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Message-ID: <20190712164531.GW26519@linux.ibm.com>
Date: Fri, 12 Jul 2019 09:45:31 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Peter Zijlstra <peterz@...radead.org>, 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, 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 11:10:51AM -0400, Joel Fernandes wrote:
> 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))
OK, I will bite... Why not also lock_is_held(&rcu_bh_lock_map)?
Thanx, Paul
> + return 1;
> + return !preemptible();
> }
> EXPORT_SYMBOL_GPL(rcu_read_lock_any_held);
>
>
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