[PATCH] hugetlb_cgroup: convert comma to semicolon

[PATCH] hugetlb_cgroup: convert comma to semicolon

[Xu Wang]

Replace a comma between expression statements by a semicolon.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
---
 mm/hugetlb_cgroup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index aabf65d4d91b..1f87aec9ab5c 100644
--- a/mm/hugetlb_cgroup.c
+++ b/mm/hugetlb_cgroup.c
@@ -655,7 +655,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
 	snprintf(cft->name, MAX_CFTYPE_NAME, "%s.events", buf);
 	cft->private = MEMFILE_PRIVATE(idx, 0);
 	cft->seq_show = hugetlb_events_show;
-	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
+	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
 	cft->flags = CFTYPE_NOT_ON_ROOT;
 
 	/* Add the events.local file */
@@ -664,7 +664,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
 	cft->private = MEMFILE_PRIVATE(idx, 0);
 	cft->seq_show = hugetlb_events_local_show;
 	cft->file_offset = offsetof(struct hugetlb_cgroup,
-				    events_local_file[idx]),
+				    events_local_file[idx]);
 	cft->flags = CFTYPE_NOT_ON_ROOT;
 
 	/* NULL terminate the last cft */
-- 
2.17.1

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Andrew Morton]

On Tue, 18 Aug 2020 06:43:33 +0000 Xu Wang <vulab@iscas.ac.cn> wrote:

> Replace a comma between expression statements by a semicolon.
> 
> ...
>
> --- a/mm/hugetlb_cgroup.c
> +++ b/mm/hugetlb_cgroup.c
> @@ -655,7 +655,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
>  	snprintf(cft->name, MAX_CFTYPE_NAME, "%s.events", buf);
>  	cft->private = MEMFILE_PRIVATE(idx, 0);
>  	cft->seq_show = hugetlb_events_show;
> -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
> +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
>  	cft->flags = CFTYPE_NOT_ON_ROOT;
>  
>  	/* Add the events.local file */
> @@ -664,7 +664,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
>  	cft->private = MEMFILE_PRIVATE(idx, 0);
>  	cft->seq_show = hugetlb_events_local_show;
>  	cft->file_offset = offsetof(struct hugetlb_cgroup,
> -				    events_local_file[idx]),
> +				    events_local_file[idx]);
>  	cft->flags = CFTYPE_NOT_ON_ROOT;
>  
>  	/* NULL terminate the last cft */

Fixes: faced7e0806cf4 ("mm: hugetlb controller for cgroups v2")

Wow, why does this code even work.  Presumably the initial value of
cft->file_offset simply doesn't matter.  Giuseppe, could you please
check?  We might have some unneeded code in there.

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Giuseppe Scrivano]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Tue, 18 Aug 2020 06:43:33 +0000 Xu Wang <vulab@iscas.ac.cn> wrote:
>
>> Replace a comma between expression statements by a semicolon.
>> 
>> ...
>>
>> --- a/mm/hugetlb_cgroup.c
>> +++ b/mm/hugetlb_cgroup.c
>> @@ -655,7 +655,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
>>  	snprintf(cft->name, MAX_CFTYPE_NAME, "%s.events", buf);
>>  	cft->private = MEMFILE_PRIVATE(idx, 0);
>>  	cft->seq_show = hugetlb_events_show;
>> -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
>> +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
>>  	cft->flags = CFTYPE_NOT_ON_ROOT;
>>  
>>  	/* Add the events.local file */
>> @@ -664,7 +664,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
>>  	cft->private = MEMFILE_PRIVATE(idx, 0);
>>  	cft->seq_show = hugetlb_events_local_show;
>>  	cft->file_offset = offsetof(struct hugetlb_cgroup,
>> -				    events_local_file[idx]),
>> +				    events_local_file[idx]);
>>  	cft->flags = CFTYPE_NOT_ON_ROOT;
>>  
>>  	/* NULL terminate the last cft */
>
> Fixes: faced7e0806cf4 ("mm: hugetlb controller for cgroups v2")

Xu, thanks for spotting it.  Was this code causing any issue or have you
found by inspecting it?

> Wow, why does this code even work.  Presumably the initial value of
> cft->file_offset simply doesn't matter.  Giuseppe, could you please
> check?  We might have some unneeded code in there.

I think in this case having two expressions as part of the same
statement is equivalent to having two separate statements.  Both
cft->file_offset and cft->flags get the expected value.

Regards,
Giuseppe

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Giuseppe Scrivano]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Tue, 18 Aug 2020 06:43:33 +0000 Xu Wang <vulab@iscas.ac.cn> wrote:
>
>> Replace a comma between expression statements by a semicolon.
>> 
>> ...
>>
>> --- a/mm/hugetlb_cgroup.c
>> +++ b/mm/hugetlb_cgroup.c
>> @@ -655,7 +655,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
>>  	snprintf(cft->name, MAX_CFTYPE_NAME, "%s.events", buf);
>>  	cft->private = MEMFILE_PRIVATE(idx, 0);
>>  	cft->seq_show = hugetlb_events_show;
>> -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
>> +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
>>  	cft->flags = CFTYPE_NOT_ON_ROOT;
>>  
>>  	/* Add the events.local file */
>> @@ -664,7 +664,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
>>  	cft->private = MEMFILE_PRIVATE(idx, 0);
>>  	cft->seq_show = hugetlb_events_local_show;
>>  	cft->file_offset = offsetof(struct hugetlb_cgroup,
>> -				    events_local_file[idx]),
>> +				    events_local_file[idx]);
>>  	cft->flags = CFTYPE_NOT_ON_ROOT;
>>  
>>  	/* NULL terminate the last cft */
>
> Fixes: faced7e0806cf4 ("mm: hugetlb controller for cgroups v2")

Xu, thanks for spotting it.  Was this code causing any issue or have you
found by inspecting it?

> Wow, why does this code even work.  Presumably the initial value of
> cft->file_offset simply doesn't matter.  Giuseppe, could you please
> check?  We might have some unneeded code in there.

I think in this case having two expressions as part of the same
statement is equivalent to having two separate statements.  Both
cft->file_offset and cft->flags get the expected value.

Regards,
Giuseppe

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Matthew Wilcox]

On Wed, Aug 19, 2020 at 10:14:11AM +0200, Giuseppe Scrivano wrote:
> >> -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
> >> +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
> >>  	cft->flags = CFTYPE_NOT_ON_ROOT;
> 
> I think in this case having two expressions as part of the same
> statement is equivalent to having two separate statements.  Both
> cft->file_offset and cft->flags get the expected value.

That's not how the comma operator works.

It will evaluate offsetof(struct hugetlb_cgroup, events_file[idx]) and
then discard the result.  Since it has no side-effects, this is effectively
doing:

	cft->file_offset = cft->flags = CFTYPE_NOT_ON_ROOT;

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Matthew Wilcox]

On Sun, Aug 23, 2020 at 04:21:30PM +0100, Matthew Wilcox wrote:
> On Wed, Aug 19, 2020 at 10:14:11AM +0200, Giuseppe Scrivano wrote:
> > >> -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
> > >> +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
> > >>  	cft->flags = CFTYPE_NOT_ON_ROOT;
> > 
> > I think in this case having two expressions as part of the same
> > statement is equivalent to having two separate statements.  Both
> > cft->file_offset and cft->flags get the expected value.
> 
> That's not how the comma operator works.
> 
> It will evaluate offsetof(struct hugetlb_cgroup, events_file[idx]) and
> then discard the result.  Since it has no side-effects, this is effectively
> doing:
> 
> 	cft->file_offset = cft->flags = CFTYPE_NOT_ON_ROOT;

_oh_.  I tested this.  I'm wrong because the comma operator is at lower
precedence than assignment.

Testcase:

struct a {
  int x;
  int y;
};

void g(struct a *a) {
  a->x = 1,
  a->y = 0;
}

void h(struct a *a) {
  a->x = (1,
  a->y = 0);
}

test.c: In function ‘h’:
test.c:12:12: warning: left-hand operand of comma expression has no effect [-Wunused-value]
   12 |   a->x = (1,
      |            ^

0000000000000000 <g>:
   0:	48 c7 07 01 00 00 00 	movq   $0x1,(%rdi)
   7:	c3                   	retq   
   8:	0f 1f 84 00 00 00 00 	nopl   0x0(%rax,%rax,1)
   f:	00 

0000000000000010 <h>:
  10:	48 c7 07 00 00 00 00 	movq   $0x0,(%rdi)
  17:	c3                   	retq   

So there's no bug here!  It's just confusing, so should be fixed.

(I think Andrew was confused too ;-)

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Joe Perches]

On Sun, 2020-08-23 at 16:21 +0100, Matthew Wilcox wrote:
> On Wed, Aug 19, 2020 at 10:14:11AM +0200, Giuseppe Scrivano wrote:
> > > > -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
> > > > +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
> > > >  	cft->flags = CFTYPE_NOT_ON_ROOT;
> > 
> > I think in this case having two expressions as part of the same
> > statement is equivalent to having two separate statements.  Both
> > cft->file_offset and cft->flags get the expected value.
> 
> That's not how the comma operator works.
> 
> It will evaluate offsetof(struct hugetlb_cgroup, events_file[idx]) and
> then discard the result.  Since it has no side-effects, this is effectively
> doing:
> 
> 	cft->file_offset = cft->flags = CFTYPE_NOT_ON_ROOT;

$ gcc -x c -
#include <stdio.h>
#include <stdlib.h>

struct foo {
	int a;
	char b[50];
};

int main(int argc, char **argv)
{
	int a;
	int b;

	a = sizeof(struct foo), b = 1;

	printf("a: %d, b: %d\n", a, b);

	return 0;
}
$ ./a.out
a: 56, b: 1

Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

[Joe Perches]

On Sun, 2020-08-23 at 16:21 +0100, Matthew Wilcox wrote:
> On Wed, Aug 19, 2020 at 10:14:11AM +0200, Giuseppe Scrivano wrote:
> > > > -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
> > > > +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
> > > >  	cft->flags = CFTYPE_NOT_ON_ROOT;
> > 
> > I think in this case having two expressions as part of the same
> > statement is equivalent to having two separate statements.  Both
> > cft->file_offset and cft->flags get the expected value.
> 
> That's not how the comma operator works.
> 
> It will evaluate offsetof(struct hugetlb_cgroup, events_file[idx]) and
> then discard the result.  Since it has no side-effects, this is effectively
> doing:
> 
> 	cft->file_offset = cft->flags = CFTYPE_NOT_ON_ROOT;

$ gcc -x c -
#include <stdio.h>
#include <stdlib.h>

struct foo {
	int a;
	char b[50];
};

int main(int argc, char **argv)
{
	int a;
	int b;

	a = sizeof(struct foo), b = 1;

	printf("a: %d, b: %d\n", a, b);

	return 0;
}
$ ./a.out
a: 56, b: 1