Messages in this thread | | | From | Anshuman Khandual <> | Subject | Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() | Date | Thu, 21 May 2020 08:45:38 +0530 |
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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@arm.com> >>>> Cc: Will Deacon <will@kernel.org> >>>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com> >>>> Cc: Mark Brown <broonie@kernel.org> >>>> Cc: linux-arm-kernel@lists.infradead.org >>>> Cc: linux-kernel@vger.kernel.org >>>> >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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). >> >> 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
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