Re: [PATCH 1/2] count_stat_eventual: Switch from ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@gmail.com>
> >>> ---
> >>>  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; })

> 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();
> >>>
> >>
> > 
> > 
>