Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg()

[Anshuman Khandual]

On 05/20/2020 11:09 PM, Will Deacon wrote:
> On Wed, May 20, 2020 at 04:47:11PM +0100, Catalin Marinas wrote:
>> On Wed, May 20, 2020 at 01:20:13PM +0100, Will Deacon wrote:
>>> 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@xxxxxxx>
>>>> Cc: Will Deacon <will@xxxxxxxxxx>
>>>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>>> Cc: Mark Brown <broonie@xxxxxxxxxx>
>>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>>>
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>>> ---
>>>> 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).
>>
>> Such WARN can be triggered by the user via emulate_sys_reg(), so we
>> can't really have it in get_arm64_ftr_reg() without a 'failsafe' option.
> 
> Ah yes, that would be bad. In which case, I don't think the existing code
> should change.

The existing code has BUG_ON() in three different callers doing exactly the
same thing that can easily be taken care in get_arm64_ftr_reg() itself. As
mentioned before an enum variable (as preferred - over a bool) can still
preserve the existing behavior for emulate_sys_reg().

IMHO these are very good reasons for us to change the code which will make
it cleaner while also removing three redundant BUG_ON() instances. Hence I
will request you to please reconsider this proposal.

- Anshuman