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

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Date:	Mon, 8 Nov 2010 16:06:47 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Joe Korty <joe.korty@...r.com>, mathieu.desnoyers@...icios.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 12:28:12PM -0700, Paul E. McKenney wrote:
> On Fri, Nov 05, 2010 at 05:00:59PM -0400, Joe Korty wrote:
> > +/**
> > + * synchronize_sched - block until all CPUs have exited any non-preemptive
> > + * kernel code sequences.
> > + *
> > + * This means that all preempt_disable code sequences, including NMI and
> > + * hardware-interrupt handlers, in progress on entry will have completed
> > + * before this primitive returns.  However, this does not guarantee that
> > + * softirq handlers will have completed, since in some kernels
> 
> OK, so your approach treats preempt_disable code sequences as RCU
> read-side critical sections by relying on the fact that the per-CPU
> ->krcud task cannot run until such code sequences complete, correct?
> 
> This seems to require that each CPU's ->krcud task be awakened at
> least once per grace period, but I might well be missing something.



I understood it differently, but I might also be wrong as well. krcud
executes the callbacks, but it is only woken up for CPUs that want to
execute callbacks, not for those that only signal a quiescent state,
which is only determined in two ways through rcu_poll_other_cpus():

- if the CPU is in an rcu_read_lock() critical section, it has the
  IN_RCU_READ_LOCK flag. If so then we set up its DO_RCU_COMPLETION flag so
  that it signals its quiescent state on rcu_read_unlock().

- otherwise it's in a quiescent state.


This works for rcu and rcu bh critical sections.
But this works in rcu sched critical sections only if rcu_read_lock_sched() has
been called explicitly, otherwise that doesn't work (in preempt_disable(),
local_irq_save(), etc...). I think this is what is not complete when
Joe said it's not yet a complete rcu implementation.

This is also the part that scaries me most :)

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