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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Fri, Nov 05, 2010 at 05:00:59PM -0400, Joe Korty wrote:
[...]
> > + *
> > + * RCU read-side critical sections may be nested.  Any deferred actions
> > + * will be deferred until the outermost RCU read-side critical section
> > + * completes.
> > + *
> > + * It is illegal to block while in an RCU read-side critical section.
> > + */
> > +void __rcu_read_lock(void)
> > +{
> > +	struct rcu_data *r;
> > +
> > +	r = &per_cpu(rcu_data, smp_processor_id());
> > +	if (r->nest_count++ == 0)
> > +		/*
> > +		 * Set the flags value to show that we are in
> > +		 * a read side critical section.  The code starting
> > +		 * a batch uses this to determine if a processor
> > +		 * needs to participate in the batch.  Including
> > +		 * a sequence allows the remote processor to tell
> > +		 * that a critical section has completed and another
> > +		 * has begun.
> > +		 */
> > +		r->flags = IN_RCU_READ_LOCK | (r->sequence++ << 2);
> 
> It seems to me that we need a memory barrier here -- what am I missing?

Agreed, I spotted it too. One more is needed, see below,

> 
> > +}
> > +EXPORT_SYMBOL(__rcu_read_lock);
> > +
> > +/**
> > + * 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.

> > +	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