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

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Date:   Wed, 27 Oct 2021 18:05:09 +0200
From:   Antoine Tenart <atenart@...nel.org>
To:     David Ahern <dsahern@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Russell King <linux@...linux.org.uk>, davem@...emloft.net
Cc:     Maxime Chevallier <maxime.chevallier@...tlin.com>,
        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

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?

> 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.

Antoine