netdev - Re: [PATCH net-next 4/4] sch_netem: replace spin_(un)lock_bh with sch_tree

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Date:	Thu, 13 Feb 2014 17:46:06 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	yangyingliang@...wei.com
Cc:	netdev@...r.kernel.org, stephen@...workplumber.org
Subject: Re: [PATCH net-next 4/4] sch_netem: replace spin_(un)lock_bh with
 sch_tree_(un)lock

From: Yang Yingliang <yangyingliang@...wei.com>
Date: Wed, 12 Feb 2014 10:58:15 +0800

> spin_(un)lock_bh(root_lock) is same as sch_tree_(un)lock.
> 
> Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
 ...
> @@ -684,11 +683,9 @@ static int get_dist_table(struct Qdisc *sch, const struct nlattr *attr)
>  	for (i = 0; i < n; i++)
>  		d->table[i] = data[i];
>  
> -	root_lock = qdisc_root_sleeping_lock(sch);
> -
> -	spin_lock_bh(root_lock);
> +	sch_tree_lock(sch);
>  	swap(q->delay_dist, d);
> -	spin_unlock_bh(root_lock);
> +	sch_tree_unlock(sch);
>  
>  	dist_free(d);
>  	return 0;

This is more expensive than the existing code.

We will now calculate qdisc_root_sleeping_lock() twice which is at
least two pointer dereferences each.

Without explicitly open-coding this, the compiler cannot cache the
result, because the spin lock operations have memory barriers (if
implemented inline) or are considered to potentially modify all memory
(if implemented as function calls).
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