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    SubjectRe: [PATCH tip/core/rcu 03/28] rcu: Streamline code produced by __rcu_read_unlock()
    * Paul E. McKenney (paulmck@linux.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

    >
    > 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.

    Other than that, feel free to add my

    Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

    >
    > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.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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