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    SubjectRe: [PATCH RFC tip/core/rcu 02/16] rcu: make dead code really dead
    On Sat, Apr 17, 2010 at 06:12:13PM -0700, Paul E. McKenney wrote:
    > On Fri, Apr 16, 2010 at 09:53:27PM -0700, Josh Triplett wrote:
    > > On Fri, Apr 16, 2010 at 03:29:27PM -0700, Paul E. McKenney wrote:
    > > > On Fri, Apr 16, 2010 at 02:16:10PM -0700, Josh Triplett wrote:
    > > > > 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. :)
    > > >
    > > > Actually, the "if" condition is a comparison of numerical constants,
    > > > so no #ifdef is required.
    > >
    > > I just meant that those constants get #defined based on CONFIG_NO_HZ, so
    > > an #ifdef on that would remove the need for the special handling of dead
    > > code.
    >
    > True, I could put a #ifdef CONFIG_NO_HZ around that leg of the switch
    > statement. Or am I still missing your point?

    No, you got it exactly. Hence my suggesting that all the other
    alternatives (the if with a break, or with __builtin_unreachable) seemed
    excessive just to try to convince the compiler to infer what an ifdef
    would tell it explicitly. :)

    - Josh Triplett


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