Messages in this thread | | | Date | Sat, 6 Nov 2010 15:34:56 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] a local-timer-free version of RCU |
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* 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
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