linux-kernel - Re: [PATCH v2 net-next 1/7] bpf: add 'flags' attribute to BPF_MAP_UPDATE

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Date:	Fri, 14 Nov 2014 07:33:00 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 1/7] bpf: add 'flags' attribute to
 BPF_MAP_UPDATE_ELEM command

On Fri, Nov 14, 2014 at 4:11 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Do, 2014-11-13 at 17:36 -0800, Alexei Starovoitov wrote:
>> the current meaning of BPF_MAP_UPDATE_ELEM syscall command is:
>> either update existing map element or create a new one.
>> Initially the plan was to add a new command to handle the case of
>> 'create new element if it didn't exist', but 'flags' style looks
>> cleaner and overall diff is much smaller (more code reused), so add 'flags'
>> attribute to BPF_MAP_UPDATE_ELEM command with the following meaning:
>>  #define BPF_ANY      0 /* create new element or update existing */
>>  #define BPF_NOEXIST  1 /* create new element if it didn't exist */
>>  #define BPF_EXIST    2 /* update existing element */
>
> Would a cmpxchg-alike function be handy here?

you mean cmpxchg command in addition to
update() command ?
May be... it will have an extra 'value' argument
(key, old_value, new_value)
I don't have a use case for it yet though.
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