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SubjectRe: [PATCH] Documentation/memory-barriers: fix wrong comment in example
On Sun, Feb 21, 2016 at 07:50:19AM +0900, SeongJae Park wrote:
> On Sun, Feb 21, 2016 at 4:57 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Sat, Feb 20, 2016 at 03:01:08PM +0900, SeongJae Park wrote:
> >> There is wrong comment in example for compiler store omit behavior. It
> >> shows example of the problem and than problem solved version code.
> >> However, the comment in the solved version is still same with not solved
> >> version. Fix the wrong statement with this commit.
> >>
> >> Signed-off-by: SeongJae Park <sj38.park@gmail.com>
> >
> > Hmmm... The code between the two stores of zero to "a" is intended to
> > remain the same in the broken and fixed versions. So the only change
> > is from "a = 0" to "WRITE_ONCE(a, 0)". Note that it is some other
> > CPU that did the third store to "a".
>
> Agree, of course.
>
> >
> > Or am I missing your point here?
>
> My point is about the comment.
> I thought the comment in broken version is saying "Below line(a = 0) says
> it will store to variable 'a', but it will not in actual because a compiler can
> omit it".
> However, in fixed version, because the compiler cannot omit the store
> now, I thought the comment also should be changed to say the difference
> between broken and fixed version.
>
> If I am understanding anything wrong, please let me know.

Hmmm... The intent of the comment is to act as a placeholder for
arbitrary code that does not affect the value of "a". The current
comment is clearly not doing that for you. Possible changes include:

o Adding test to the comment making the intent more clear.
o Replacing the comment with a function call, perhaps to
does_not_change_a() or some similar name.
o Keeping the current comment, but adding a call to something
like does_not_change_a() after it.

Other thoughts?

Thanx, Paul

> Thanks,
> SeongJae Park
>
> >
> > Thanx, Paul
> >
> >> ---
> >> Documentation/memory-barriers.txt | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> >> index 061ff29..b4754c7 100644
> >> --- a/Documentation/memory-barriers.txt
> >> +++ b/Documentation/memory-barriers.txt
> >> @@ -1471,7 +1471,7 @@ of optimizations:
> >> wrong guess:
> >>
> >> WRITE_ONCE(a, 0);
> >> - /* Code that does not store to variable a. */
> >> + /* Code that does store to variable a. */
> >> WRITE_ONCE(a, 0);
> >>
> >> (*) The compiler is within its rights to reorder memory accesses unless
> >> --
> >> 1.9.1
> >>
> >
>

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