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?
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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