Re: A question about QuickQuiz 5.46

Hi all,

I have a comment on this old thread.

On 2023/05/21 19:42, Akira Yokosawa wrote:
> On Sun, 21 May 2023 07:54:25 +0000, Hao Lee wrote:
>> On Fri, May 19, 2023 at 10:27:48PM +0900, Akira Yokosawa wrote:
>>> On Fri, 19 May 2023 06:28:47 +0000, Hao Lee wrote:
>>>> On Thu, May 18, 2023 at 10:50:51AM +0900, Akira Yokosawa wrote:
>>>>> Hi,
>>>>>
>>>>> On Wed, 17 May 2023 12:01:10 +0000, Hao Lee wrote:
>>>>>> Hi, Paul
>>>>>>
>>>>>> I noticed that QuickQuiz 5.46 is causing confusion due to the fact that
>>>>>> balance_count() and flush_local_count() are both located within the
>>>>>> slowpath and are called sequentially. As a result, it is impossible for
>>>>>> them to compete for the counterandmax thread-local variable.
>>>>>
>>>>> Right.
>>>>> And that's exactly what is mentioned in the answer to the quiz.
>>>>>
>>>>> I guess it confused you because you didn't expect such an easy one,
>>>>> no ? :-)
>>>>
>>>> In fact I'm confused about the question description.
>>>> I believe that this quiz should focus on the concurrency of global
>>>> variables, rather than on the concurrency of the atomic variable
>>>> `counterandmax`, which is currently the main focus of the description.
>>>
>>> Hmm, sorry but I'm afraid I don't get your point ...
>>>
>>> Could you share the question you'd expect to see in place of
>>> Quick Quiz 5.46?
>>
>> Hi, I don't think this question should exist according to my
>> understanding...
>>
>> Let's approach this question from a different perspective. The answer
>> emphasizes that the gblcnt_mutex prevents concurrency problems. Let's
>> assume that neither balance_count() nor flush_local_count() is holding
>> the gblcnt_mutex. Would the counterandmax variable encounter concurrency
>> problems? I don't think so because counterandmax is an atomic variable
>> and is always safe, even without using any locks.
> 
> I'm not sure, but you might be missing subtle wording in the quiz part
> of Quick Quiz 5.46 (quoted below):
> 
>     Given that the atomic_set() primitive does a simple store
>     to the specified atomic_t, ...
> 
> Here, "simple store" means a normal store which doesn't involve
> any atomic operations (at instruction level).
> 
> atomic_read() and atomic_set() are for accessing atomic_t variables
> without any costly atomic operations.

This was obvious up until commit cee469f9a7aa ("api: Switch x86 atomics
to gcc C11-like atomics").

Before the change, atomic_read() and atomic_set() were defined in
CodeSamples/api.h as:

    typedef struct { volatile int counter; } atomic_t;

    #define atomic_read(v)         ((v)->counter)
    #define atomic_set(v, i)       (((v)->counter) = (i))

Now, they are defined in CodeSamples/api-pthreads/api-gcc.h as:

    typedef struct { volatile int counter; } atomic_t;

    #define atomic_read(v) \
           __atomic_load_n(&(v)->counter, __ATOMIC_RELAXED)
    #define atomic_set(v, i) \
           __atomic_store_n(&(v)->counter, (i), __ATOMIC_RELAXED)

It is the C compiler's call what instructions to use for accessing
atomic variables.  Therefore, what Hao said above (quoted again for
your convenience):

>>                                                                 Let's
>> assume that neither balance_count() nor flush_local_count() is holding
>> the gblcnt_mutex. Would the counterandmax variable encounter concurrency
>> problems? I don't think so because counterandmax is an atomic variable
>> and is always safe, even without using any locks.

is actually quite true.

Now I think QQz 5.46 needs at least some rewording.

Hao, apologies for my missing the change under CodeSamples/ at the time.

Paul, what do you think?

        Thanks, Akira

> 
> Which is why Quick Quiz 5.46 has been there for more than a decade.
> 
> Perhaps, you might be interested in Quick Quiz B.12 as well.
>  
> HTH,
>         Akira
> 
>>
>>>
>>>         Thanks, Akira
>>>
>>>>
>>>>>
>>>>>         Thanks, Akira
>>>>>
>>>>>>                                                               Therefore,
>>>>>> I believe that this quiz may be referring to the global variables
>>>>>> globalcount and globalreserve.
>>>>>>
>>>>>> Thanks,
>>>>>> Hao Lee