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

[Paul E. McKenney]

On Mon, Feb 22, 2016 at 11:16:39AM +, David Howells wrote:
> SeongJae Park  wrote:
> 
> > From f7b5677790771599f418f1d95536935be971ae86 Mon Sep 17 00:00:00 2001
> > From: SeongJae Park 
> > Date: Mon, 22 Feb 2016 19:26:18 +0900
> > Subject: [PATCH] Documentation/memory-barriers: polish compiler store omit
> >  example
> > 
> > Comments of examples about compiler store omit in memory-barriers.txt is
> > about code that could be possible at that point.  However, someone could
> > interpret the comment as an explanation about below line.  This commit
> > exploits the intent more explicitly by changing the comment to be seems
> > like a possible code rather than explanation about below line.
> > 
> > Signed-off-by: SeongJae Park 
> > ---
> >  Documentation/memory-barriers.txt | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/memory-barriers.txt
> > b/Documentation/memory-barriers.txt
> > index 904ee42..dc66351 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1459,7 +1459,7 @@ of optimizations:
> >   the following:
> > 
> > a = 0;
> > -   /* Code that does not store to variable a. */
> > +   ... Code that does not store to variable a ...
> > a = 0;
> > 
> >   The compiler sees that the value of variable 'a' is already zero, so
> > @@ -1471,7 +1471,7 @@ of optimizations:
> >   wrong guess:
> > 
> > WRITE_ONCE(a, 0);
> > -   /* Code that does not store to variable a. */
> > +   ... Code that does not store to variable a ...
> > WRITE_ONCE(a, 0);
> > 
> >   (*) The compiler is within its rights to reorder memory accesses unless
> 
> Acked-by: David Howells 

Thank you both!  Patch with updated commit log below, please let me know
if you have any objections to the changes.

Thanx, Paul



commit 0a41feb6ab4da3218192e2cde1a54fcc5d8f5658
Author: SeongJae Park 
Date:   Mon Feb 22 08:28:29 2016 -0800

documentation: Clarify compiler store-fusion example

The compiler store-fusion example in memory-barriers.txt uses a C
comment to represent arbitrary code that does not update a given
variable.  Unfortunately, someone could reasonably interpret the
comment as instead referring to the following line of code.  This
commit therefore replaces the comment with a string that more
clearly represents the arbitrary code.

Signed-off-by: SeongJae Park 
Acked-by: David Howells 
Signed-off-by: Paul E. McKenney 

diff --git a/Documentation/memory-barriers.txt 
b/Documentation/memory-barriers.txt
index 8367d393cba2..3729cbe60e41 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1550,7 +1550,7 @@ of optimizations:
  the following:
 
a = 0;
-   /* Code that does not store to variable a. */
+   ... Code that does not store to variable a ...
a = 0;
 
  The compiler sees that the value of variable 'a' is already zero, so
@@ -1562,7 +1562,7 @@ of optimizations:
  wrong guess:
 
WRITE_ONCE(a, 0);
-   /* Code that does not store to variable a. */
+   ... Code that does not store to variable a ...
WRITE_ONCE(a, 0);
 
  (*) The compiler is within its rights to reorder memory accesses unless

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

[SeongJae Park]

On Mon, 22 Feb 2016, David Howells wrote:


SeongJae Park  wrote:


  a = 0;
  /* Code that does not store to variable a. */
+ does_not_change_a();
  a = 0;


Since it's not actually code that's meant to be executed, you could make it:

a = 0;
... code that does not store to variable a ...
a = 0;


I selected Paul's third option because the function could be noop (In this
case, it doesn't break the original meaning) and it makes the code looks
complete.
However, your suggestion looks much better than the comment, too.

So, I am attaching a patch that applying your suggestion below.


=== >3 

From f7b5677790771599f418f1d95536935be971ae86 Mon Sep 17 00:00:00 2001

From: SeongJae Park 
Date: Mon, 22 Feb 2016 19:26:18 +0900
Subject: [PATCH] Documentation/memory-barriers: polish compiler store omit
 example

Comments of examples about compiler store omit in memory-barriers.txt is
about code that could be possible at that point.  However, someone could
interpret the comment as an explanation about below line.  This commit
exploits the intent more explicitly by changing the comment to be seems
like a possible code rather than explanation about below line.

Signed-off-by: SeongJae Park 
---
 Documentation/memory-barriers.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/memory-barriers.txt 
b/Documentation/memory-barriers.txt

index 904ee42..dc66351 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1459,7 +1459,7 @@ of optimizations:
  the following:

a = 0;
-   /* Code that does not store to variable a. */
+   ... Code that does not store to variable a ...
a = 0;

  The compiler sees that the value of variable 'a' is already zero, so
@@ -1471,7 +1471,7 @@ of optimizations:
  wrong guess:

WRITE_ONCE(a, 0);
-   /* Code that does not store to variable a. */
+   ... Code that does not store to variable a ...
WRITE_ONCE(a, 0);

  (*) The compiler is within its rights to reorder memory accesses unless
--
1.9.1





David


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

[David Howells]

SeongJae Park  wrote:

>   a = 0;
>   /* Code that does not store to variable a. */
> + does_not_change_a();
>   a = 0;

Since it's not actually code that's meant to be executed, you could make it:

a = 0;
... code that does not store to variable a ...
a = 0;

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

[Paul E. McKenney]

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

[SeongJae Park]

On Sun, Feb 21, 2016 at 4:57 AM, Paul E. McKenney
 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 
>
> 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.


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

[Paul E. McKenney]

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 

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".

Or am I missing your point here?

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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