linux-kernel - Re: [PATCH] a local-timer-free version of RCU

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Message-ID: <20101106194408.GK15561@linux.vnet.ibm.com>
Date:	Sat, 6 Nov 2010 12:44:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Joe Korty <joe.korty@...r.com>, fweisbec@...il.com,
	dhowells@...hat.com, loic.minier@...aro.org,
	dhaval.giani@...il.com, tglx@...utronix.de, peterz@...radead.org,
	linux-kernel@...r.kernel.org, josh@...htriplett.org
Subject: Re: [PATCH] a local-timer-free version of RCU

On Sat, Nov 06, 2010 at 03:42:19PM -0400, Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (mathieu.desnoyers@...icios.com) wrote:
> > > > +/**
> > > > + * rcu_read_unlock - marks the end of an RCU read-side critical section.
> > > > + * Check if a RCU batch was started while we were in the critical
> > > > + * section.  If so, call rcu_quiescent() join the rendezvous.
> > > > + *
> > > > + * See rcu_read_lock() for more information.
> > > > + */
> > > > +void __rcu_read_unlock(void)
> > > > +{
> > > > +	struct rcu_data *r;
> > > > +	int	cpu, flags;
> > > > +
> > 
> > Another memory barrier would be needed here to ensure that the memory accesses
> > performed within the C.S. are not reordered wrt nest_count decrement.
> 
> Nevermind. xchg() acts as a memory barrier, and nest_count is only ever touched
> by the local CPU. No memory barrier needed here.

You beat me to it.  ;-)

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> > > > +	cpu = smp_processor_id();
> > > > +	r = &per_cpu(rcu_data, cpu);
> > > > +	if (--r->nest_count == 0) {
> > > > +		flags = xchg(&r->flags, 0);
> > > > +		if (flags & DO_RCU_COMPLETION)
> > > > +			rcu_quiescent(cpu);
> > > > +	}
> > > > +}
> > > > +EXPORT_SYMBOL(__rcu_read_unlock);
> > 
> > Thanks,
> > 
> > Mathieu
> > 
> > -- 
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
> --
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