linux-kernel - RE: [PATCH v5 1/8] x86/resctrl: Prepare for new domain scope

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Message-ID: <SJ1PR11MB6083112BEC62D2AB6F432EFFFCE5A@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date:   Thu, 31 Aug 2023 17:26:23 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>
CC:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        "Jonathan Corbet" <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        "Babu Moger" <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: RE: [PATCH v5 1/8] x86/resctrl: Prepare for new domain scope

> >I've sometimes had compilers complain about code written:
> >
> >static int get_domain_id_from_scope(int cpu, enum resctrl_scope scope)
> >{
> >        switch (scope) {
> >        case RESCTRL_L3_CACHE:
> >                return get_cpu_cacheinfo_id(cpu, 3);
> >        case RESCTRL_L2_CACHE:
> >                return get_cpu_cacheinfo_id(cpu, 2);
> >        default:
> >                WARN_ON_ONCE(1);
> >                return -1;
> >        }
> >}
> >
> >because they failed to notice that every path in the switch does a "return and they
> >issue a warning that the function has no return value because they don't realize
> >that the end of the function can't be reached.
> >
> >So it's defensive programming against possible complier issues.
>
> I recall getting that error somewhere while playing around with a
> language server protocol for neovim a while ago but I tried to cause
> it today with gcc and clang and with some different flags and coulnd't.
> Are there some particular compilers or compiler flags that trigger
> that?

I think I saw this from an lkp report using clang. But it's quite possible
that the exact code construct was different in some way.

-Tony