linux-kernel - Re: [PATCH RFC tip/core/rcu 02/16] rcu: make dead code really dead

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Message-ID: <20100416211610.GA8357@feather>
Date:	Fri, 16 Apr 2010 14:16:10 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 02/16] rcu: make dead code really dead

On Fri, Apr 16, 2010 at 07:23:48AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 15, 2010 at 04:52:52PM -0700, Josh Triplett wrote:
> > On Thu, Apr 15, 2010 at 11:13:25AM -0700, Paul E. McKenney wrote:
> > > From: Lai Jiangshan <laijs@...fujitsu.com>
> > > 
> > > cleanup: make dead code really dead
> > > 
> > > Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > ---
> > >  kernel/rcutree.c |    4 ++--
> > >  1 files changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index e54c123..6042fb8 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -1236,11 +1236,11 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
> > >  		break; /* grace period idle or initializing, ignore. */
> > >  
> > >  	case RCU_SAVE_DYNTICK:
> > > -
> > > -		raw_spin_unlock(&rnp->lock);  /* irqs remain disabled */
> > >  		if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
> > >  			break; /* So gcc recognizes the dead code. */
> > 
> > GCC's new __builtin_unreachable would help here, though obviously we
> > can't count on 4.5 or newer quite yet.  A wrapper in compiler.h would
> > let us use it when available though.
> 
> So at some time when we can count on gcc 4.5 or newer, the code
> would look something like the following?
> 
> 	if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK)
> 		this_is_unreachable();

Yes, exactly.

> I suppose that in the meantime one could supply the code to use
> in the unreachable case:
> 
> 	if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK)
> 		this_is_unreachable(break);
> 
> But this is beginning to seem a bit strained to me.  ;-)

I'd suggest spelling that this way:

	if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) {
		unreachable();
		break;
	}

But in any case, all of these do seem excessive just to avoid the need
for an ifdef. :)

- Josh Triplett
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