linux-kernel - Re: [PATCH RFC 6/9] RCU priority boosting for preemptible RCU

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Date:	Fri, 5 Oct 2007 08:24:21 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Gautham R Shenoy <ego@...ibm.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	dipankar@...ibm.com, josht@...ux.vnet.ibm.com, tytso@...ibm.com,
	dvhltc@...ibm.com, Thomas Gleixner <tglx@...utronix.de>,
	a.p.zijlstra@...llo.nl, bunk@...nel.org, oleg@...sign.ru
Subject: Re: [PATCH RFC 6/9] RCU priority boosting for preemptible RCU




On Fri, 5 Oct 2007, Gautham R Shenoy wrote:

> On Mon, Sep 10, 2007 at 11:39:01AM -0700, Paul E. McKenney wrote:
>
> [snip]
>
> > +
> > +/*
> > + * Return the list from which to boost target tasks.
> > + * May only be invoked by the booster task, so guaranteed to
> > + * already be initialized.  Use rcu_boost_dat element least recently
> > + * the destination for task blocking in RCU read-side critical sections.
> > + */
> > +static inline struct rcu_boost_dat *rcu_rbd_boosting(int cpu)
> > +{
> > +	int idx = (rcu_boost_idx + 1) & (RCU_BOOST_ELEMENTS - 1);
>
> Why is this masking required? When we increment
> the rcu_boost_idx in rcu_booster, we do perform a modulo operation
> to ensure that it wraps around RCU_BOOST_ELEMENTS.

Because we are not masking rcu_boost_idx, we are masking
 (rcu_boost_idx + 1) which may extend the bounderies of
RCU_BOOST_ELEMENTS.

-- Steve
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