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SubjectRe: [RFC PATCH net] net: ipconfig: Release the rtnl_lock while waiting for carrier
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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

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