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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 06/09/2011 03:29 AM, Paul E. McKenney 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.
> 
> 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 */

Since ACCESS_ONCE() is used for loading ->rcu_read_unlock_special, is the previous
barrier() still needed?

> +		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 */