[PATCH v3] arm64: kernel: implement fast refcount checking

[Li Kun]

在 2017/7/31 21:59, Ard Biesheuvel 写道:
> On 31 July 2017 at 14:17, Li Kun <hw.likun at huawei.com> wrote:
>>
>> 在 2017/7/31 20:01, Ard Biesheuvel 写道:
>>> +static __always_inline __must_check bool refcount_add_not_zero(unsigned
>>> int i,
>>> +                                                              refcount_t
>>> *r)
>>> +{
>>> +       unsigned long tmp;
>>> +       int result;
>>> +
>>> +       asm volatile("// refcount_add_not_zero \n"
>>> +"      prfm            pstl1strm, %2\n"
>>> +"1:    ldxr            %w[val], %2\n"
>>> +"      cbz             %w[val], 2f\n"
>>> +"      adds            %w[val], %w[val], %w[i]\n"
>>> +"      stxr            %w1, %w[val], %2\n"
>>> +"      cbnz            %w1, 1b\n"
>>> +       REFCOUNT_POST_CHECK_NEG
>>> +"2:"
>>> +       : [val] "=&r" (result), "=&r" (tmp), "+Q" (r->refs.counter)
>>> +       : REFCOUNT_INPUTS(&r->refs) [i] "Ir" (i)
>>> +       : REFCOUNT_CLOBBERS);
>>> +
>>> +       return result != 0;
>>> +}
>>> +
>> Could we use "cas" here instead of ll/sc ?
>>
> I don't see how, to be honest.
>
> Compare and swap only performs the store if the expected value is in
> the memory location. In this case, we don't know the old value, we
> only know we need to do something special if it is 0.
I think you are right:)
I thought wrongly, we could get the old value only after the 'cas' has 
excuted successfully.
Thanks a lot.

-- 
Best Regards
Li Kun