netdev - Re: [RFC PATCH net] net: ipconfig: Release the rtnl

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Message-ID: <20211028084520.4b96f93a@bootlin.com>
Date:   Thu, 28 Oct 2021 08:45:20 +0200
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     Antoine Tenart <atenart@...nel.org>
Cc:     David Ahern <dsahern@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com
Subject: Re: [RFC PATCH net] net: ipconfig: Release the rtnl_lock while
 waiting for carrier

Hello Antoine,

On Wed, 27 Oct 2021 18:05:09 +0200
Antoine Tenart <atenart@...nel.org> wrote:

>Hi Maxime,
>
>Quoting Maxime Chevallier (2021-10-27 15:19:53)
>> While waiting for a carrier to come on one of the netdevices, some
>> devices will require to take the rtnl lock at some point to fully
>> initialize all parts of the link.
>> 
>> That's the case for SFP, where the rtnl is taken when a module gets
>> detected. This prevents mounting an NFS rootfs over an SFP link.
>> 
>> This means that while ipconfig waits for carriers to be detected, no SFP
>> modules can be detected in the meantime, it's only detected after
>> ipconfig times out.
>> 
>> This commit releases the rtnl_lock while waiting for the carrier to come
>> up, and re-takes it to check the for the init device and carrier status.
>> 
>> At that point, the rtnl_lock seems to be only protecting
>> ic_is_init_dev().
>> 
>> Fixes: 73970055450e ("sfp: add SFP module support")  
>
>Was this working with SFP modules before?

>From what I can tell, no. In that case, does it need a fixes tag ?
It seems the problem has always been there, and booting an nfsroot
never worked over SFP links.

>
>> diff --git a/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
>> index 816d8aad5a68..069ae05bd0a5 100644
>> --- a/net/ipv4/ipconfig.c
>> +++ b/net/ipv4/ipconfig.c
>> @@ -278,7 +278,12 @@ static int __init ic_open_devs(void)
>>                         if (ic_is_init_dev(dev) && netif_carrier_ok(dev))
>>                                 goto have_carrier;
>>  
>> +               /* Give a chance to do complex initialization that
>> +                * would require to take the rtnl lock.
>> +                */
>> +               rtnl_unlock();
>>                 msleep(1);
>> +               rtnl_lock();
>>  
>>                 if (time_before(jiffies, next_msg))
>>                         continue;  
>
>The rtnl lock is protecting 'for_each_netdev' and 'dev_change_flags' in
>this function. What could happen in theory is a device gets removed from
>the list or has its flags changed. I don't think that's an issue here.
>
>Instead of releasing the lock while sleeping, you could drop the lock
>before the carrier waiting loop (with a similar comment) and only
>protect the above 'for_each_netdev' loop.

Nice catch, the effect should be the same but with a much cleaner idea
of what is being protected.

I'll give it a try and respin, thanks for the review !

Maxime

>Antoine



-- 
Maxime Chevallier, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com