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

[Cong Wang <xiyou.wangcong@gmail.com>]

On Fri, Aug 10, 2018 at 3:29 AM Vlad Buslov <vladbu@mellanox.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. ;)