On 2017/05/14 7:56, Paul E. McKenney wrote:
> On Sat, May 13, 2017 at 11:37:06PM +0900, Akira Yokosawa wrote:
>> On 2017/05/13 05:45:56 -0700, Paul E. McKenney wrote:
>>> On Sat, May 13, 2017 at 09:04:38PM +0900, Akira Yokosawa wrote:
>>>> Hi Jason & Paul,
>>>>
>>>> although this has already been applied, I have a comment.
>>>>
>>>> On 2017/05/11 23:03:41 +0800, Junchang Wang wrote:
>>>>> Signed-off-by: Junchang Wang <junchangwang@xxxxxxxxx>
>>>>> ---
>>>>> CodeSamples/count/count_stat_eventual.c | 8 ++++----
>>>>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/CodeSamples/count/count_stat_eventual.c b/CodeSamples/count/count_stat_eventual.c
>>>>> index 059ab8b..cbde4aa 100644
>>>>> --- a/CodeSamples/count/count_stat_eventual.c
>>>>> +++ b/CodeSamples/count/count_stat_eventual.c
>>>>> @@ -27,12 +27,12 @@ int stopflag;
>>>>>
>>>>> void inc_count(void)
>>>>> {
>>>>> - ACCESS_ONCE(__get_thread_var(counter))++;
>>>>> + READ_ONCE(__get_thread_var(counter))++;
>>>>
>>>> This is OK because READ_ONCE() is defined as the same as ACCESS_ONCE()
>>>> in CodeSamples. However, the definition in the current Linux kernel
>>>> would not permit this.
>>>>
>>>> A read-modify-write access would need both READ_ONCE() and WRITE_ONCE().
>>>> However, since "counter" is thread local and updated only by its owner,
>>>> we don't need READ_ONCE() here. So:
>>>>
>>>> + WRITE_ONCE(__get_thread_var(counter), __get_thread_var(counter) + 1);
>>>>
>>>> should have been sufficient.
>>>>
>>>> Problem with this change is that the line gets too wide when applied to
>>>> the code snippet in 2-column layout.
>>>
>>> Good point -- though renumbering the code is not all -that- hard.
>>>
>>> I clearly should have made a better READ_ONCE() that enforced the same
>>> constraints as does the Linux kernel, perhaps something like this:
>>>
>>> #define READ_ONCE(x) ({ ACCESS_ONCE(x) })
>>>
>>> Thoughts?
>>
>> I assume you meant:
>>
>> #define READ_ONCE(x) ({ ACCESS_ONCE(x); })
>
> Indeed! Good catch!!!
>
>> I'm afraid this still permits uses such as:
>>
>> READ_ONCE(y)++;
>>
>> Looks like we need a complex definition which resembles that of
>> include/linux/compiler.h. Hmm???
>
> OK, how about this?
>
> #define READ_ONCE(x) ({ typeof(x) ___x = ACCESS_ONCE(x); ___x; })
Ah, this resulted in an error as expected!
main.c: In function ‘inc_test’:
main.c:11:18: error: lvalue required as increment operand
READ_ONCE(*p)++;
Glad to know we can avoid the complexity of the kernel.
Thanks, Akira
>
>> And I have another pending question regarding 2/2 of this patch set.
>> That might result in other addition of line to the code. I think
>> I can send it tomorrow.
>
> Looking forward to seeing it! ;-)
>
> Thanx, Paul
>
>>>> Hmm...
>>>>
>>>> Thanks, Akira
>>>>
>>>>> }
>>>>>
>>>>> unsigned long read_count(void)
>>>>> {
>>>>> - return ACCESS_ONCE(global_count);
>>>>> + return READ_ONCE(global_count);
>>>>> }
>>>>>
>>>>> void *eventual(void *arg)
>>>>> @@ -43,8 +43,8 @@ void *eventual(void *arg)
>>>>> while (stopflag < 3) {
>>>>> sum = 0;
>>>>> for_each_thread(t)
>>>>> - sum += ACCESS_ONCE(per_thread(counter, t));
>>>>> - ACCESS_ONCE(global_count) = sum;
>>>>> + sum += READ_ONCE(per_thread(counter, t));
>>>>> + WRITE_ONCE(global_count, sum);
>>>>> poll(NULL, 0, 1);
>>>>> if (stopflag) {
>>>>> smp_mb();
>>>>>
>>>>
>>>
>>>
>>
>
> .
>
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