linux-kernel - Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers

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Date:	Tue, 6 Oct 2015 14:05:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for
 Tiny and Tree RCU readers

On Tue, Oct 06, 2015 at 10:52:00PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 06, 2015 at 01:18:01PM -0700, Paul E. McKenney wrote:
> > On Tue, Oct 06, 2015 at 10:05:38PM +0200, Peter Zijlstra wrote:
> 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index b9d9e0249e2f..93c0f23c3e45 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
> > > >   */
> > > >  void rcu_note_context_switch(void)
> > > >  {
> > > > +	barrier(); /* Avoid RCU read-side critical sections leaking down. */
> > > >  	trace_rcu_utilization(TPS("Start context switch"));
> > > >  	rcu_sched_qs();
> > > >  	rcu_preempt_note_context_switch();
> > > >  	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
> > > >  		rcu_momentary_dyntick_idle();
> > > >  	trace_rcu_utilization(TPS("End context switch"));
> > > > +	barrier(); /* Avoid RCU read-side critical sections leaking up. */
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(rcu_note_context_switch);
> > > 
> > > These OTOH could be fixed with a noinline, such that the compiler may
> > > never inline it, even with whole-program-optimizations, thereby
> > > guaranteeing a function call boundary or compiler barrier.
> > 
> > I like the barrier() with the comment.  I expect it will be a bit more
> > robust against toolchain changes.
> 
> Don't you in fact already rely on the fact that schedule() is a function
> call and will not be inlined? (it doesn't have noinline and I suppose
> whole program optimizers could go funny on it).

Probably pretty much everywhere I call schedule().  But I was thinking
that barrier() and the beginning and end of an external function didn't
need to do anything.  Is that incorrect?

							Thanx, Paul

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