linux-kernel - Re: [PATCH tip/core/rcu 15/15] rcu: RCU_SAVE

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Message-ID: <20120616010436.GX2389@linux.vnet.ibm.com>
Date:	Fri, 15 Jun 2012 18:04:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 15/15] rcu: RCU_SAVE_DYNTICK code no longer
 ever dead

On Fri, Jun 15, 2012 at 05:04:49PM -0700, Josh Triplett wrote:
> On Fri, Jun 15, 2012 at 05:02:38PM -0700, Josh Triplett wrote:
> > On Fri, Jun 15, 2012 at 02:06:10PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > 
> > > Before RCU had unified idle, the RCU_SAVE_DYNTICK leg of the switch
> > > statement in force_quiescent_state() was dead code for CONFIG_NO_HZ=n
> > > kernel builds.  With unified idle, the code is never dead.  This commit
> > > therefore removes the "if" statement designed to make gcc aware of when
> > > the code was and was not dead.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > One comment below; with that change:
> > 
> > Reviewed-by: Josh Triplett <josh@...htriplett.org>
> > 
> > >  kernel/rcutree.c |    2 --
> > >  1 files changed, 0 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index 75ad92a..0b0c9cc 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -1744,8 +1744,6 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
> > >  		break; /* grace period idle or initializing, ignore. */
> > >  
> > >  	case RCU_SAVE_DYNTICK:
> > > -		if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
> > > -			break; /* So gcc recognizes the dead code. */
> > >  
> > >  		raw_spin_unlock(&rnp->lock);  /* irqs remain disabled */
> > 
> > Drop the blank line too?
> 
> Actually, I just realized a larger concern with what this change
> implies: does this mean that whatever change made this code no longer
> dead introduced a major locking bug here?  If so, has that change
> already progressed past the point where you could update it to include
> this fix?

No, the lock is dropped and then reacquired, so the "break" is OK.
This change should have been made back when dyntick-idle mode became
unconditional from RCU's viewpoint.

And yes, I probably should change "rcu_dyntick" to "rcu_idle" and
make a bunch of similar changes.  But not particularly high priority.

							Thanx, Paul

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