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

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Date: Fri, 12 Jul 2019 13:11:25 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Joel Fernandes (Google)" <joel@...lfernandes.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 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.

> +		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;

	return !preemptible();

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