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); })
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???
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.
Thanks, Akira
>
> 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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