netdev - Re: [net] Question about ipvs->sysctl_sync_threshold and READ

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Date: Wed, 20 Mar 2024 10:41:44 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Zijie Zhao <zzjas98@...il.com>
Cc: horms@...ge.net.au, ja@....bg, davem@...emloft.net, dsahern@...nel.org, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	netdev@...r.kernel.org, lvs-devel@...r.kernel.org, chenyuan0y@...il.com
Subject: Re: [net] Question about ipvs->sysctl_sync_threshold and READ_ONCE

On Wed, Mar 20, 2024 at 6:49 AM Zijie Zhao <zzjas98@...il.com> wrote:
>
> Dear IPVS maintainers,
>
> We encountered an unusual usage of sysctl parameter while analyzing
> kernel source code.
>
>
> In include/net/ip_vs.h, line 1062 - 1070:
>
> ```
> static inline int sysctl_sync_threshold(struct netns_ipvs *ipvs)
> {
>         return ipvs->sysctl_sync_threshold[0];
> }
>
> static inline int sysctl_sync_period(struct netns_ipvs *ipvs)
> {
>         return READ_ONCE(ipvs->sysctl_sync_threshold[1]);
> }
> ```
>
> Here, sysctl_sync_threshold[1] is accessed behind `READ_ONCE`, but
> sysctl_sync_threshold[0] is not. Should sysctl_sync_threshold[0] also be
> guarded by `READ_ONCE`?

I'm not so sure and clear about the detailed history.

AFAIK, readers accessing this sysctl knob (sysctl_sync_threshold)
should be protected by READ_ONCE() because it can be changed
concurrently. Probably the commit 749c42b620a95 missed this point many
years ago and then commit 6aa7de059173a followed that.

Thanks,
Jason

>
> Please kindly let us know if we missed any key information and this is
> actually intended. We appreciate your information and time! Thanks!
>
>
> Links to the code:
> https://elixir.bootlin.com/linux/v6.8.1/source/include/net/ip_vs.h#L1064
> https://elixir.bootlin.com/linux/v6.8.1/source/include/net/ip_vs.h#L1069
>
> Best,
> Zijie
>