netdev - Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

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Message-ID: <b370c097-d0ad-95c5-2687-264da22d37d6@cumulusnetworks.com>
Date:   Mon, 27 Mar 2017 08:43:05 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     netdev@...r.kernel.org, roopa@...ulusnetworks.com,
        rshearma@...cade.com
Subject: Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

On 3/26/17 9:11 PM, Eric W. Biederman wrote:
> I don't like this.  Byte writes don't exist on all architectures.
> 
> So while I think always writing to rtn_nhn_alive under the
> rtn_lock ensures that we don't have wrong values written
> it is quite subtle.  And I don't know how this will interact with other
> fields that you are introducing.
> 
> AKA this might be ok, but I expect this formulation of the code
> will easily bit-rot and break.

net/ has other use cases -- e.g., ipv6 tunneling has proto as a u8.

It unrealistic for a route to have 255 or more nexthops so the point of
this patch is to not waste 8 bytes tracking it - especially when
removing it gets routes with ipv4 and ipv6 via's into a cache line.

I can make the alive counter a u16 without increasing the size of the
struct. I'd prefer to leave it as an u8 to have a u8 hole for flags
should something be needed later.