netdev - Re: [PATCH net-next v6 10/11] net: sched: atomically check-allocate action

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Date:   Fri, 10 Aug 2018 14:45:24 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yevgeny Kliteynik <kliteyn@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [PATCH net-next v6 10/11] net: sched: atomically check-allocate action

On Fri, Aug 10, 2018 at 3:29 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
> Approach you suggest is valid, but has its own trade-offs:
>
> - As you noted, lock granularity becomes coarse-grained due to per-netns
> scope.

Sure, you acquire idrinfo->lock too, the only difference is how long
you take it.

The bottleneck of your approach is the same, also you take idrinfo->lock
twice, so the contention is heavier.


>
> - I am not sure it is possible to call idr_replace() without obtaining
> idrinfo->lock in this particular case. Concurrent delete of action with
> same id is possible and, according to idr_replace() description,
> unlocked execution is not supported for such use-case:

But we can hold its refcnt before releasing idrinfo->lock, so
idr_replace() can't race with concurrent delete.


>
> - High rate or replace request will generate a lot of unnecessary memory
> allocations and deallocations.
>

Yes, this is literally how RCU works, always allocate and copy,
release upon error.

Also, if this is really a problem, we have SLAB_TYPESAFE_BY_RCU
too. ;)