netdev - Re: [PATCH net-next v2 5/6] bonding: restructure and add rcu for bond_for_each_slave

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Date:	Wed, 4 Sep 2013 18:29:04 +0200
From:	Veaceslav Falico <vfalico@...hat.com>
To:	Ding Tianhong <dthxman@...il.com>
Cc:	Ding Tianhong <dingtianhong@...wei.com>,
	Jay Vosburgh <fubar@...ibm.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	"David S. Miller" <davem@...emloft.net>,
	Nikolay Aleksandrov <nikolay@...hat.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 5/6] bonding: restructure and add rcu for
 bond_for_each_slave_next()

On Wed, Sep 04, 2013 at 11:02:28PM +0800, Ding Tianhong wrote:
>于 2013/9/4 18:35, Veaceslav Falico 写道:
>>On Wed, Sep 04, 2013 at 05:44:15PM +0800, Ding Tianhong wrote:
>>...snip...
>>>+/* Check whether the slave is the only one in bond */
>>>+#define bond_is_only_slave(bond, pos) \
>>>+ (((pos)->list.prev == &(bond)->slave_list) && \
>>>+ ((pos)->list.next == &(bond)->slave_list))
>>
>>Could be done without pos at all -
>>
>>!list_empty(&(bond)->slave_list) && \
>>&(bond)->slave_list.next == &(bond)->slave_list.prev
>>
>>If we have only one slave and pos is NOT our slave then... well.. we have
>>big troubles.
>>
>yes, more simple more beautiful, thanks.
>
>but if the pos is not our slave, it is the mistake, not bug. :)
>
>>>+
>>>/**
>>>* bond_for_each_slave_from - iterate the slaves list from a 
>>>starting point
>>>* @bond: the bond holding this list.
>>>* @pos: current slave.
>>>- * @cnt: counter for max number of moves
>>>* @start: starting point.
>>>*
>>>* Caller must hold bond->lock
>>>*/
>>>-#define bond_for_each_slave_from(bond, pos, cnt, start) \
>>>- for (cnt = 0, pos = start; pos && cnt < (bond)->slave_cnt; \
>>>- cnt++, pos = bond_next_slave(bond, pos))
>>>+#define bond_for_each_slave_from(bond, pos, start) \
>>>+ for (pos = start; pos && (bond_is_only_slave(bond, start) ? \
>>>+ &pos->list != &bond->slave_list : \
>>>+ &pos->list != &start->list); bond_is_only_slave(bond, start) ? \
>>>+ (pos = list_entry(pos->list.next, typeof(*pos), list)) : \
>>>+ (pos = bond_next_slave(bond, pos)))
>>
>>Did you check that?
>>
>>pos = slave1 (bond has more than one slave);
>>pos && &pos->list != &slave1->list - false.
>>
>>We won't ever enter this loop if we have >1 slaves.
>>
>>I don't understand this at all.
>>
>
>ok, the logic is : if slaves == 1, run once for the slave.

your code, actually, says differently:

(bond_is_only_slave(bond, start) ? &pos->list != &bond->slave_list :

which means that if bond_is_only_slave(bond, start) == true then we check
&pos->list != &bond->slave_list, so we run till the end of the list.

Ternary operator works like that

expression ? what_to_do_if_true : what_to_do_if_false

http://en.wikipedia.org/wiki/?:#C

>if slaves > 1, run loops until reach the end list.

Here we test for &pos->list != &start->list, which is false cause pos ==
start.

>I could not get a better way to simplify the function, I think it is 
>a suitable scheme.

Nope. bond_for_each_slave_from() is supposed to loop through slaves from a
starting slave and till that starting slave, not included.

Your loop... I don't understand what it does. But clearly not what it was
doing (and supposed to do).

>
>by the way, I test the function and works well. :)
>
>>>+
>>>+/**
>>>+ * bond_for_each_slave_from_rcu - iterate the slaves list from a 
>>>starting point
>>>+ * @bond: the bond holding this list.
>>>+ * @pos: current slave.
>>>+ * @start: starting point.
>>>+ *
>>>+ * Caller must hold rcu_read_lock
>>>+ */
>>>+#define bond_for_each_slave_from_rcu(bond, pos, start) \
>>>+ for (pos = start; pos && (bond_is_only_slave(bond, start) ? \
>>>+ &pos->list != &bond->slave_list : \
>>>+ &pos->list != &start->list); bond_is_only_slave(bond, start) ? \
>>>+ (pos = list_entry_rcu(pos->list.next, typeof(*pos), list)) : \
>>>+ (pos = bond_next_slave_rcu(bond, pos)))
>>
>>Ditto as bond_for_each_slave_from() and, also, see my comment about RCU
>>from patch 1.
>>
>>>
>>>/**
>>>* bond_for_each_slave - iterate over all slaves
>>>-- 
>>>1.8.2.1
>>>
>>>
>>>
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