linux-kernel - Re: [RFC] msr: judge the return val of function rdmsrl_on_cpu() by WARN

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Message-Id: <20230630024549.76487-1-lizhe.67@bytedance.com>
Date:   Fri, 30 Jun 2023 10:45:49 +0800
From:   lizhe.67@...edance.com
To:     tglx@...utronix.de
Cc:     bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, lizefan.x@...edance.com,
        lizhe.67@...edance.com, mingo@...hat.com, rafael@...nel.org,
        tony.luck@...el.com, viresh.kumar@...aro.org, x86@...nel.org,
        yuanzhu@...edance.com
Subject: Re: [RFC] msr: judge the return val of function rdmsrl_on_cpu() by WARN_ON

On Fri, 30 Jun 2023 at 01:03:22 +0200, tglx@...utronix.de wrote:
>Li!
>
>On Thu, Jun 29 2023 at 15:27, lizhe.67@...edance.com wrote:
>
>> There are ten places call rdmsrl_on_cpu() in the current code without
>> judging the return value. This may introduce a potential bug. For example,
>> inj_bank_set() may return -EINVAL, show_base_frequency() may show an error
>> freq value, intel_pstate_hwp_set() may write an error value to the related
>> msr register and so on. But rdmsrl_on_cpu() do rarely returns an error, so
>> it seems that add a WARN_ON is enough for debugging.
>
>Can you please structure your changelogs as documented in:
>
>  https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog
>
>instead of providing a big lump of words?
>
>> There are ten places call rdmsrl_on_cpu() in the current code without
>> judging the return value.
>
>Return values are not judged. They are either ignored or checked/evaluated.
>
>> This may introduce a potential bug.
>
>Sure. Anything which does not check a return value from a function might
>be a bug, but you have to look at each instance whether its a bug or
>not.
>
>> For example, inj_bank_set() may return -EINVAL, show_base_frequency()
>> may show an error freq value, intel_pstate_hwp_set() may write an
>> error value to the related msr register and so on.  But
>> rdmsrl_on_cpu() do rarely returns an error, so it seems that add a
>> WARN_ON is enough for debugging.
>
>This is hillarious at best.
>
>  1) It does not matter at all whether that function returns an error rarely
>     or not.
>
>  2) Adding WARN_ON() without justification at each call site is not
>     enough. Neither for debugging nor for real world usage.
>
>You have to come up with individual patches for each callsite to add the
>WARN_ON() and in each patch you have to explain why it is justified and
>why there is no other solution, e.g. taking an error exit path.
>
>Just slapping WARN_ON()'s into the code without any deeper analysis is
>worse than the current state of the code.
>
>If you have identified a real world problem at any of these call sites
>then adding a WARN_ON() does not solve it at all.
>
>I'm looking forward to your profound anlysis of each of these "problems".
>
>Thanks,
>
>        tglx

Thanks for all your advice. I will analysis each of these "problems".