linux-kernel - Re: [PATCH RFC tip/core/rcu 02/10] rcu: make dead code really dead

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Message-ID: <20100405185624.GF2525@linux.vnet.ibm.com>
Date:	Mon, 5 Apr 2010 11:56:24 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, 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 02/10] rcu: make dead code really dead

On Mon, Apr 05, 2010 at 02:49:28PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > From: Lai Jiangshan <laijs@...fujitsu.com>
> > 
> > cleanup: make dead code really dead
> 
> Is it just me or this spinlock change is more than just a cleanup ? Or
> maybe it just needs a much more descriptive changelog.

Just a cleanup.  In the case where the "break" could be executed,
control will never reach the RCU_SAVE_DYNTICK leg of the switch statement.

That said, yes, if control -could- reach the RCU_SAVE_DYNTICK when
this "break" statement was executed, we would have had a locking
problem.  But as it is, this change just makes the dead code really
all be dead so that gcc knows not to produce the corresponding binary.

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> > 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. */
> >  
> > +		raw_spin_unlock(&rnp->lock);  /* irqs remain disabled */
> > +
> >  		/* Record dyntick-idle state. */
> >  		force_qs_rnp(rsp, dyntick_save_progress_counter);
> >  		raw_spin_lock(&rnp->lock);  /* irqs already disabled */
> > -- 
> > 1.7.0
> > 
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
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