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Message-ID: <20110608235814.GN2324@linux.vnet.ibm.com>
Date: Wed, 8 Jun 2011 16:58:14 -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, 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,
patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 03/28] rcu: Streamline code produced by
__rcu_read_unlock()
On Wed, Jun 08, 2011 at 07:17:17PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > Given some common flag combinations, particularly -Os, gcc will inline
> > rcu_read_unlock_special() despite its being in an unlikely() clause.
> > Use noline to prohibit this misoptimization.
>
> noline -> noinline
Good eyes, fixed!
> > In addition, move the second barrier() in __rcu_read_unlock() so that
> > it is not on the common-case code path. This will allow the compiler to
> > generate better code for the common-case path through __rcu_read_unlock().
> >
> > Finally, fix up whitespace in kernel/lockdep.c to keep checkpatch happy.
>
> This cleanup probably moved to a separate patch, but this comment line
> did not follow.
Indeed -- I have removed it.
> Other than that, feel free to add my
>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Thank you!
Thanx, Paul
> > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> > kernel/rcutree_plugin.h | 12 ++++++------
> > 1 files changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index ea2e2fb..40a6db7 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -284,7 +284,7 @@ static struct list_head *rcu_next_node_entry(struct task_struct *t,
> > * notify RCU core processing or task having blocked during the RCU
> > * read-side critical section.
> > */
> > -static void rcu_read_unlock_special(struct task_struct *t)
> > +static noinline void rcu_read_unlock_special(struct task_struct *t)
> > {
> > int empty;
> > int empty_exp;
> > @@ -387,11 +387,11 @@ void __rcu_read_unlock(void)
> > struct task_struct *t = current;
> >
> > barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */
> > - --t->rcu_read_lock_nesting;
> > - barrier(); /* decrement before load of ->rcu_read_unlock_special */
> > - if (t->rcu_read_lock_nesting == 0 &&
> > - unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> > - rcu_read_unlock_special(t);
> > + if (--t->rcu_read_lock_nesting == 0) {
> > + barrier(); /* decr before ->rcu_read_unlock_special load */
> > + if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> > + rcu_read_unlock_special(t);
> > + }
> > #ifdef CONFIG_PROVE_LOCKING
> > WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
> > #endif /* #ifdef CONFIG_PROVE_LOCKING */
> > --
> > 1.7.3.2
> >
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
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