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SubjectRe: [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@cn.fujitsu.com>
> > >
> > > cleanup: make dead code really dead
> > >
> > > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.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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