linux-kernel - Re: [PATCH] Documentation/memory-barriers: fix wrong comment in example

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Message-ID: <20160221052542.GJ3522@linux.vnet.ibm.com>
Date:	Sat, 20 Feb 2016 21:25:42 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	SeongJae Park <sj38.park@...il.com>
Cc:	Jonathan Corbet <corbet@....net>, dhowells@...hat.com,
	linux-doc <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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@...ux.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@...il.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
> >>
> >
>