netdev - Re: [PATCH bpf-next 2/8] xdp: simplify cpumap cleanup

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Date:   Wed, 18 Dec 2019 09:47:23 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf@...r.kernel.org, davem@...emloft.net, hawk@...nel.org,
        john.fastabend@...il.com, magnus.karlsson@...el.com,
        jonathan.lemon@...il.com
Subject: Re: [PATCH bpf-next 2/8] xdp: simplify cpumap cleanup

On Wed, 18 Dec 2019 11:53:54 +0100, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> After the RCU flavor consolidation [1], call_rcu() and
> synchronize_rcu() waits for preempt-disable regions (NAPI) in addition
> to the read-side critical sections. As a result of this, the cleanup
> code in cpumap can be simplified
> 
> * There is no longer a need to flush in __cpu_map_entry_free, since we
>   know that this has been done when the call_rcu() callback is
>   triggered.
> 
> * When freeing the map, there is no need to explicitly wait for a
>   flush. It's guaranteed to be done after the synchronize_rcu() call
>   in cpu_map_free().
> 
> [1] https://lwn.net/Articles/777036/
> 
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>

Probably transient but:

../kernel/bpf/cpumap.c: In function "cpu_map_free":
../kernel/bpf/cpumap.c:502:6: warning: unused variable "cpu" [-Wunused-variable]
  502 |  int cpu;
      |      ^~~

I think there are also warnings in patch 4.