Re: [PATCH net-next] net: stmmac: allow changing the MTU while the interface is running

[David Miller]

From: Niklas Cassel <niklas.cassel@xxxxxxxx>
Date: Tue, 4 Apr 2017 14:18:54 +0200

> From: Niklas Cassel <niklas.cassel@xxxxxxxx>
> 
> Setting ethtool ops for stmmac is only allowed when the interface is up.
> Setting MTU (a netdev op) for stmmac is only allowed when the interface
> is down.
> 
> It seems that the only reason why MTU cannot be changed when running is
> that we have not bothered to implement a nice way to dealloc/alloc the
> descriptor rings.
> 
> To make it less confusing for the user, call ndo_stop() and ndo_open()
> from ndo_change_mtu(). This is not a nice way to dealloc/alloc the
> descriptor rings, since it will announce that the interface is being
> brought down/up to user space, but there are several other drivers doing
> it this way, and it is arguably better than just returning -EBUSY.
> 
> Signed-off-by: Niklas Cassel <niklas.cassel@xxxxxxxx>

You can't do this with no error handling.

Instead, you must do this using a "prepare", "commit" sequence.
First making sure you can reallocate all necessary resources,
and make the config change, before actually doing so.

You're not even checking if the re-open fails, meaning that an MTU
change can cause the interface to shut down.  That is simply not
acceptable.