Subject: Re: [PATCH net] net: phy: Decrement phy_fixed_addr during unregister

From: Russell King - ARM Linux <[email-masked]>
Date: Fri, 24 Jun 2016 23:55:27 +0100

On Fri, Jun 24, 2016 at 03:44:11PM -0700, Florian Fainelli wrote:
> If we have a system which uses fixed PHY devices and calls
> fixed_phy_register() then fixed_phy_unregister() we can exhaust the
> number of fixed PHYs available after a while, since we keep incrementing
> the variable phy_fixed_addr, but we never decrement it.
> 
> This patch fixes that by decrementing phy_fixed_addr during
> fixed_phy_del(), and in order to do that, we need to move the
> phy_fixed_addr integer and its spinlock above that function.

Is this really a good idea?

What if we have two fixed phys register, and the first one is
unregistered and a new one subsequently registered?

First phy registered, gets address 0, phy_fixed_addr becomes 1.
Second phy registered, gets address 1, phy_fixed_addr becomes 2.
First phy is unregistered, phy_fixed_addr becomes 1.
Third phy registered, gets address 1, conflicts with the second phy.

Obviously not a good outcome.

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