linux-kernel - Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr

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Date:   Wed, 20 May 2020 13:20:13 +0100
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        Catalin Marinas <catalin.marinas@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside
 get_arm64_ftr_reg()

Hi Anshuman,

On Wed, May 20, 2020 at 06:52:54AM +0530, Anshuman Khandual wrote:
> There is no way to proceed when requested register could not be searched in
> arm64_ftr_reg[]. Requesting for a non present register would be an error as
> well. Hence lets just BUG_ON() when the search fails in get_arm64_ftr_reg()
> rather than checking for return value and doing the same in some individual
> callers.
> 
> But there are some callers that dont BUG_ON() upon search failure. It adds
> an argument 'failsafe' that provides required switch between callers based
> on whether they could proceed or not.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
> Applies on next-20200518 that has recent cpufeature changes from Will.
> 
>  arch/arm64/kernel/cpufeature.c | 26 +++++++++++++-------------
>  1 file changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index bc5048f152c1..62767cc540c3 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -557,7 +557,7 @@ static int search_cmp_ftr_reg(const void *id, const void *regp)
>   *         - NULL on failure. It is upto the caller to decide
>   *	     the impact of a failure.
>   */
> -static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id)
> +static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id, bool failsafe)

Generally, I'm not a big fan of boolean arguments because they are really
opaque at the callsite. It also seems bogus to me that we don't trust the
caller to pass a valid sys_id, but we trust it to get "failsafe" right,
which seems to mean "I promise to check the result isn't NULL before
dereferencing it."

So I don't see how this patch improves anything. I'd actually be more
inclined to stick a WARN() in get_arm64_ftr_reg() when it returns NULL and
have the callers handle NULL by returning early, getting rid of all the
BUG_ONs in here. Sure, the system might end up in a funny state, but we
WARN()d about it and tried to keep going (and Linus has some strong opinions
on this too).

Will