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    Subject[PATCH tip/core/rcu 03/28] rcu: Streamline code produced by __rcu_read_unlock()
    Date
    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.

    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.

    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


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