netdev - Re: [PATCH net] fib_rules: return 0 directly if an exactly same rule exists when NLM_F

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Date:   Wed, 5 Jun 2019 12:43:44 +0900
From:   Lorenzo Colitti <lorenzo@...gle.com>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     David Ahern <dsahern@...il.com>,
        David Miller <davem@...emloft.net>,
        Yaro Slav <yaro330@...il.com>,
        Thomas Haller <thaller@...hat.com>,
        Alistair Strachan <astrachan@...gle.com>,
        Greg KH <greg@...ah.com>,
        Linux NetDev <netdev@...r.kernel.org>,
        David Ahern <dsa@...ulusnetworks.com>,
        Maciej Żenczykowski <zenczykowski@...il.com>
Subject: Re: [PATCH net] fib_rules: return 0 directly if an exactly same rule
 exists when NLM_F_EXCL not supplied

On Wed, Jun 5, 2019 at 12:29 PM Hangbin Liu <liuhangbin@...il.com> wrote:
> > We rely on being able to add a rule and either have a dup be created
> > (in which case we'll remove it later) or have it fail with EEXIST (in
> > which case we won't remove it later).
>
> With Maciej said, how about add NLM_F_EXCL flag when you add a new rule.
> If it returned EEXIST, which means there is an dup rule, you just do not
> remove it later.
>
> Would that fix your issue?

We can't do that without rewriting our code and making it more
complex. The way the code is structured is that an update is "add all
new rules; delete all old rules". To do what you suggest we would need
to either change that to "for rule in rules; add newrule; delete
oldrule" or we'd need to keep state on which rules already existed.

The previous behaviour provided semantics that are useful to
userspace, and this commit broke those semantics. Please revert.