linux-kernel - Re: [PATCH] bonding: avoid repeated display of same link status change

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Date:   Tue, 18 Sep 2018 07:00:05 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Manish Kumar Singh <mk.singh@...cle.com>,
        Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bonding: avoid repeated display of same link status
 change



On 09/17/2018 10:05 PM, Manish Kumar Singh wrote:
> 
> 
>> -----Original Message-----
>> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
>> Sent: 17 सितम्बर 2018 20:08
>> To: Manish Kumar Singh; netdev@...r.kernel.org
>> Cc: Jay Vosburgh; Veaceslav Falico; Andy Gospodarek; David S. Miller; linux-
>> kernel@...r.kernel.org
>> Subject: Re: [PATCH] bonding: avoid repeated display of same link status
>> change
>>
>>
>>
>> On 09/17/2018 12:20 AM, mk.singh@...cle.com wrote:
>>> From: Manish Kumar Singh <mk.singh@...cle.com>
>>>
>>> When link status change needs to be committed and rtnl lock couldn't be
>>> taken, avoid redisplay of same link status change message.
>>>
>>> Signed-off-by: Manish Kumar Singh <mk.singh@...cle.com>
>>> ---
>>>  drivers/net/bonding/bond_main.c | 6 ++++--
>>>  include/net/bonding.h           | 1 +
>>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/bonding/bond_main.c
>> b/drivers/net/bonding/bond_main.c
>>> index 217b790d22ed..fb4e3aff1677 100644
>>> --- a/drivers/net/bonding/bond_main.c
>>> +++ b/drivers/net/bonding/bond_main.c
>>> @@ -2087,7 +2087,7 @@ static int bond_miimon_inspect(struct bonding
>> *bond)
>>>  			bond_propose_link_state(slave, BOND_LINK_FAIL);
>>>  			commit++;
>>>  			slave->delay = bond->params.downdelay;
>>> -			if (slave->delay) {
>>> +			if (slave->delay && !bond->rtnl_needed) {
>>>  				netdev_info(bond->dev, "link status down for
>> %sinterface %s, disabling it in %d ms\n",
>>>  					    (BOND_MODE(bond) ==
>>>  					     BOND_MODE_ACTIVEBACKUP) ?
>>> @@ -2127,7 +2127,7 @@ static int bond_miimon_inspect(struct bonding
>> *bond)
>>>  			commit++;
>>>  			slave->delay = bond->params.updelay;
>>>
>>> -			if (slave->delay) {
>>> +			if (slave->delay && !bond->rtnl_needed) {
>>>  				netdev_info(bond->dev, "link status up for
>> interface %s, enabling it in %d ms\n",
>>>  					    slave->dev->name,
>>>  					    ignore_updelay ? 0 :
>>> @@ -2301,9 +2301,11 @@ static void bond_mii_monitor(struct
>> work_struct *work)
>>>  		if (!rtnl_trylock()) {
>>>  			delay = 1;
>>>  			should_notify_peers = false;
>>> +			bond->rtnl_needed = true;
>>
>> How can you set a shared variable with no synchronization ?
> Thanks Eric for reviewing the patch. rtnl_needed is not a shared variable, it is part of bonding structure, that is one per bonding driver instance. There can't be two parallel instances of bond_miimon_inspect for a single  bonding driver instance at any given point of time. and only bond_miimon_inspect updates it. That’s why I think there is no need of any synchronization here.  
> 
>

If rtnl_trylock() can not grab RTNL, 
there is no way the current thread can set the  variable without a race, if the word including rtnl_needed is shared by other fields in the structure.

Your patch adds a subtle possibility of future bugs, even if it runs fine today.

Do not pave the way for future bugs, make your code robust, please.