Patchwork [19/29] batman-adv: fix random jitter calculation

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Submitter Akinobu Mita
Date Dec. 24, 2012, 2:14 a.m.
Message ID <1356315256-6572-20-git-send-email-akinobu.mita@gmail.com>
Download mbox | patch
Permalink /patch/208014/
State Not Applicable
Delegated to: David Miller
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Comments

Akinobu Mita - Dec. 24, 2012, 2:14 a.m.
batadv_iv_ogm_emit_send_time() attempts to calculates a random integer
in the range of 'orig_interval +- BATADV_JITTER' by the below lines.

        msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
        msecs += (random32() % 2 * BATADV_JITTER);

But it actually gets 'orig_interval' or 'orig_interval - BATADV_JITTER'
because '%' and '*' have same precedence and associativity is
left-to-right.

This adds the parentheses at the appropriate position so that it matches
original intension.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Marek Lindner <lindner_marek@yahoo.de>
Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Cc: Antonio Quartulli <ordex@autistici.org>
Cc: b.a.t.m.a.n@lists.open-mesh.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
---
 net/batman-adv/bat_iv_ogm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
Antonio Quartulli - Dec. 25, 2012, 11:26 a.m.
On Mon, Dec 24, 2012 at 11:14:06AM +0900, Akinobu Mita wrote:
> batadv_iv_ogm_emit_send_time() attempts to calculates a random integer
> in the range of 'orig_interval +- BATADV_JITTER' by the below lines.
> 
>         msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
>         msecs += (random32() % 2 * BATADV_JITTER);
> 
> But it actually gets 'orig_interval' or 'orig_interval - BATADV_JITTER'
> because '%' and '*' have same precedence and associativity is
> left-to-right.
> 
> This adds the parentheses at the appropriate position so that it matches
> original intension.
> 
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> Cc: Marek Lindner <lindner_marek@yahoo.de>
> Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
> Cc: Antonio Quartulli <ordex@autistici.org>
> Cc: b.a.t.m.a.n@lists.open-mesh.org
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: netdev@vger.kernel.org
> ---

Acked-by: Antonio Quartulli <ordex@autistici.org>

But I would suggest to apply this change to net, since it is a fix.

Cheers,
Akinobu Mita - Dec. 25, 2012, 9:35 p.m.
2012/12/25 Antonio Quartulli <ordex@autistici.org>:
> On Mon, Dec 24, 2012 at 11:14:06AM +0900, Akinobu Mita wrote:
>> batadv_iv_ogm_emit_send_time() attempts to calculates a random integer
>> in the range of 'orig_interval +- BATADV_JITTER' by the below lines.
>>
>>         msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
>>         msecs += (random32() % 2 * BATADV_JITTER);
>>
>> But it actually gets 'orig_interval' or 'orig_interval - BATADV_JITTER'
>> because '%' and '*' have same precedence and associativity is
>> left-to-right.
>>
>> This adds the parentheses at the appropriate position so that it matches
>> original intension.
>>
>> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>> Cc: Marek Lindner <lindner_marek@yahoo.de>
>> Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
>> Cc: Antonio Quartulli <ordex@autistici.org>
>> Cc: b.a.t.m.a.n@lists.open-mesh.org
>> Cc: "David S. Miller" <davem@davemloft.net>
>> Cc: netdev@vger.kernel.org
>> ---
>
> Acked-by: Antonio Quartulli <ordex@autistici.org>
>
> But I would suggest to apply this change to net, since it is a fix.

I agree.
David, please consider to apply this patch for 3.8-rc*.
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David Miller - Dec. 25, 2012, 9:37 p.m.
From: Akinobu Mita <akinobu.mita@gmail.com>
Date: Wed, 26 Dec 2012 06:35:37 +0900

> 2012/12/25 Antonio Quartulli <ordex@autistici.org>:
>> On Mon, Dec 24, 2012 at 11:14:06AM +0900, Akinobu Mita wrote:
>>> batadv_iv_ogm_emit_send_time() attempts to calculates a random integer
>>> in the range of 'orig_interval +- BATADV_JITTER' by the below lines.
>>>
>>>         msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
>>>         msecs += (random32() % 2 * BATADV_JITTER);
>>>
>>> But it actually gets 'orig_interval' or 'orig_interval - BATADV_JITTER'
>>> because '%' and '*' have same precedence and associativity is
>>> left-to-right.
>>>
>>> This adds the parentheses at the appropriate position so that it matches
>>> original intension.
>>>
>>> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>>> Cc: Marek Lindner <lindner_marek@yahoo.de>
>>> Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
>>> Cc: Antonio Quartulli <ordex@autistici.org>
>>> Cc: b.a.t.m.a.n@lists.open-mesh.org
>>> Cc: "David S. Miller" <davem@davemloft.net>
>>> Cc: netdev@vger.kernel.org
>>> ---
>>
>> Acked-by: Antonio Quartulli <ordex@autistici.org>
>>
>> But I would suggest to apply this change to net, since it is a fix.
> 
> I agree.
> David, please consider to apply this patch for 3.8-rc*.
> 

All patches I should consider seriously should be properly
reposted for review and inclusion.
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Patch

diff --git a/net/batman-adv/bat_iv_ogm.c b/net/batman-adv/bat_iv_ogm.c
index 9f3925a..7d02ebd 100644
--- a/net/batman-adv/bat_iv_ogm.c
+++ b/net/batman-adv/bat_iv_ogm.c
@@ -123,7 +123,7 @@  batadv_iv_ogm_emit_send_time(const struct batadv_priv *bat_priv)
 	unsigned int msecs;
 
 	msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
-	msecs += (random32() % 2 * BATADV_JITTER);
+	msecs += random32() % (2 * BATADV_JITTER);
 
 	return jiffies + msecs_to_jiffies(msecs);
 }