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From: Cong Wang <xiyou.wangcong@gmail.com>
To: Yahui Chen <goodluckwillcomesoon@gmail.com>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>, zhangwei123171@jd.com
Subject: Re: can add a new bpf helper function bpf_map_compare_and_update_elem?
Date: Fri, 26 Nov 2021 12:25:10 -0800	[thread overview]
Message-ID: <YaFCplDzX3NjN3iB@unknown> (raw)
In-Reply-To: <CAPydje8FKWzRCR33RanGZkucavFZNb2zSGhfQdrd49Uvgc2YxQ@mail.gmail.com>

On Thu, Nov 25, 2021 at 04:34:48PM +0800, Yahui Chen wrote:
> Suppose we have a map, MAP_A, and the user program does the following:
> 
> 1. bpf_map_lookup_elem(MAP_A, key, value)
> 2. change the value
> 3. bpf_map_update_elem(MAP_A, key, value, FLAG)
> 
> At the same time, the kernel's BPF program may also be modifying the value.
> 
> Then we have concurrency problems.
> 

This is why we have bpf spinlock. ;)


> Therefore, can we add a helper function like compare and swap?
> 

I don't think you can atomically compare and swap values larger than
CPU word size.


Thanks.

  reply	other threads:[~2021-11-26 20:30 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-25  8:34 can add a new bpf helper function bpf_map_compare_and_update_elem? Yahui Chen
2021-11-26 20:25 ` Cong Wang [this message]
2021-11-29  6:49   ` Yahui Chen

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