[PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Guenter Roeck]

Commit 263a523 fixes a warning seen with W=1 due to change in
DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
with unsigned divisors to unsigned, even if the dividend is signed and
negative (for example, -10 / 5U = 858993457). The C standard says "If one
operand has unsigned int type, the other operand is converted to unsigned
int", so the compiler is not to blame.
As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
bad values, since the automatic conversion of expressions such as "0 - 2U/2"
to unsigned was not taken into account.

Fix by checking for the divisor variable type when deciding which operation
to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
for negative dividends divided by unsigned divisors. Mark the latter case as
unsupported.

Reported-by: Juergen Beisert <jbe@pengutronix.de>
Tested-by: Juergen Beisert <jbe@pengutronix.de>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v2: Description update (v1 wasn't supposed to make it to lkml)

 include/linux/kernel.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index d97ed58..45726dc 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -77,13 +77,15 @@
 
 /*
  * Divide positive or negative dividend by positive divisor and round
- * to closest integer. Result is undefined for negative divisors.
+ * to closest integer. Result is undefined for negative divisors and
+ * for negative dividends if the divisor variable type is unsigned.
  */
 #define DIV_ROUND_CLOSEST(x, divisor)(			\
 {							\
 	typeof(x) __x = x;				\
 	typeof(divisor) __d = divisor;			\
-	(((typeof(x))-1) > 0 || (__x) > 0) ?		\
+	(((typeof(x))-1) > 0 ||				\
+	 ((typeof(divisor))-1) > 0 || (__x) > 0) ?	\
 		(((__x) + ((__d) / 2)) / (__d)) :	\
 		(((__x) - ((__d) / 2)) / (__d));	\
 }							\
-- 
1.7.9.7

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Andrew Morton]

On Wed, 19 Dec 2012 06:40:15 -0800
Guenter Roeck <linux@roeck-us.net> wrote:

> Commit 263a523 fixes a warning seen with W=1 due to change in
> DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> with unsigned divisors to unsigned, even if the dividend is signed and
> negative (for example, -10 / 5U = 858993457). The C standard says "If one
> operand has unsigned int type, the other operand is converted to unsigned
> int", so the compiler is not to blame.
> As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> to unsigned was not taken into account.
> 
> Fix by checking for the divisor variable type when deciding which operation
> to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> for negative dividends divided by unsigned divisors. Mark the latter case as
> unsupported.

The changelog didn't describe the end-user visible effects of the bug. 
Please always include this information.  Because...

The patch is applicable to 3.7.x.  Should we backport it?

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Jean Delvare]

Hi Guenter,

On Wed, 19 Dec 2012 06:40:15 -0800, Guenter Roeck wrote:
> Commit 263a523 fixes a warning seen with W=1 due to change in
> DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> with unsigned divisors to unsigned, even if the dividend is signed and
> negative (for example, -10 / 5U = 858993457). The C standard says "If one
> operand has unsigned int type, the other operand is converted to unsigned
> int", so the compiler is not to blame.

This is surprising to say the least. But if the C standard says so...

I wouldn't be surprised if there are bugs because of this in the kernel
and in other projects.

> As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> to unsigned was not taken into account.
> 
> Fix by checking for the divisor variable type when deciding which operation
> to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> for negative dividends divided by unsigned divisors. Mark the latter case as
> unsupported.

True but this last issue isn't specific to the DIV_ROUND_CLOSEST
implementation, it would also happen with a simple division.

> Reported-by: Juergen Beisert <jbe@pengutronix.de>
> Tested-by: Juergen Beisert <jbe@pengutronix.de>
> Cc: Jean Delvare <khali@linux-fr.org>
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
> v2: Description update (v1 wasn't supposed to make it to lkml)
> 
>  include/linux/kernel.h |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index d97ed58..45726dc 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -77,13 +77,15 @@
>  
>  /*
>   * Divide positive or negative dividend by positive divisor and round
> - * to closest integer. Result is undefined for negative divisors.
> + * to closest integer. Result is undefined for negative divisors and
> + * for negative dividends if the divisor variable type is unsigned.

Thinking a bit more about this... Documenting the non-working cases is
great, however I don't really expect all developers to pay attention. I
can also imagine variable types changing from signed to unsigned later,
and never thinking this can introduce a bug.

So, is there nothing we can do to spot at least the second issue at
build time? For regular division there's nothing we can do (although I
don't understand why gcc doesn't warn...) but here we get the
opportunity to report the issue, let's take it.

And given that the divisor is almost always a constant,
maybe we can check for negative divisors too, this would be safer and
the code size increase would probably be very small in practice.
Opinions?

>   */
>  #define DIV_ROUND_CLOSEST(x, divisor)(			\
>  {							\
>  	typeof(x) __x = x;				\
>  	typeof(divisor) __d = divisor;			\
> -	(((typeof(x))-1) > 0 || (__x) > 0) ?		\
> +	(((typeof(x))-1) > 0 ||				\
> +	 ((typeof(divisor))-1) > 0 || (__x) > 0) ?	\
>  		(((__x) + ((__d) / 2)) / (__d)) :	\
>  		(((__x) - ((__d) / 2)) / (__d));	\
>  }							\

Looks good.

-- 
Jean Delvare

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Guenter Roeck]

On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> On Wed, 19 Dec 2012 06:40:15 -0800
> Guenter Roeck <linux@roeck-us.net> wrote:
> 
> > Commit 263a523 fixes a warning seen with W=1 due to change in
> > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> > with unsigned divisors to unsigned, even if the dividend is signed and
> > negative (for example, -10 / 5U = 858993457). The C standard says "If one
> > operand has unsigned int type, the other operand is converted to unsigned
> > int", so the compiler is not to blame.
> > As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> > bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> > to unsigned was not taken into account.
> > 
> > Fix by checking for the divisor variable type when deciding which operation
> > to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> > for negative dividends divided by unsigned divisors. Mark the latter case as
> > unsupported.
> 
> The changelog didn't describe the end-user visible effects of the bug. 
> Please always include this information.  Because...
> 
One observed effect is that the s2c_hwmon driver reports a value of 4198403
instead of 0 if the ADC reads 0.

Other impact is unpredictable. Problem is seen if the divisor is an unsigned
variable or constant and the dividend is less than (divisor/2).

> The patch is applicable to 3.7.x.  Should we backport it?
> 
Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
unpredictable.

3.6 needs it as well.

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Guenter Roeck]

On Wed, Dec 19, 2012 at 11:21:15PM +0100, Jean Delvare wrote:
> Hi Guenter,
> 
> On Wed, 19 Dec 2012 06:40:15 -0800, Guenter Roeck wrote:
> > Commit 263a523 fixes a warning seen with W=1 due to change in
> > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> > with unsigned divisors to unsigned, even if the dividend is signed and
> > negative (for example, -10 / 5U = 858993457). The C standard says "If one
> > operand has unsigned int type, the other operand is converted to unsigned
> > int", so the compiler is not to blame.
> 
> This is surprising to say the least. But if the C standard says so...
> 
Agreed, but it is how it is.

> I wouldn't be surprised if there are bugs because of this in the kernel
> and in other projects.
> 
Might easily be. This might make a good interview question - I suspect many
if not most engineers would fail it. At least I would have until yesterday :).

> > As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> > bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> > to unsigned was not taken into account.
> > 
> > Fix by checking for the divisor variable type when deciding which operation
> > to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> > for negative dividends divided by unsigned divisors. Mark the latter case as
> > unsupported.
> 
> True but this last issue isn't specific to the DIV_ROUND_CLOSEST
> implementation, it would also happen with a simple division.
> 
Correct, which is why I did not try to fix it. Still worth mentioning, though,
in my opinion.

> > Reported-by: Juergen Beisert <jbe@pengutronix.de>
> > Tested-by: Juergen Beisert <jbe@pengutronix.de>
> > Cc: Jean Delvare <khali@linux-fr.org>
> > Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> > ---
> > v2: Description update (v1 wasn't supposed to make it to lkml)
> > 
> >  include/linux/kernel.h |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index d97ed58..45726dc 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -77,13 +77,15 @@
> >  
> >  /*
> >   * Divide positive or negative dividend by positive divisor and round
> > - * to closest integer. Result is undefined for negative divisors.
> > + * to closest integer. Result is undefined for negative divisors and
> > + * for negative dividends if the divisor variable type is unsigned.
> 
> Thinking a bit more about this... Documenting the non-working cases is
> great, however I don't really expect all developers to pay attention. I
> can also imagine variable types changing from signed to unsigned later,
> and never thinking this can introduce a bug.
> 
> So, is there nothing we can do to spot at least the second issue at
> build time? For regular division there's nothing we can do (although I
> don't understand why gcc doesn't warn...) but here we get the
> opportunity to report the issue, let's take it.
> 
> And given that the divisor is almost always a constant,
> maybe we can check for negative divisors too, this would be safer and
> the code size increase would probably be very small in practice.
> Opinions?
> 
Agreed, though we should fix the problem now and think about reporting
afterwards.

Guenter

> >   */
> >  #define DIV_ROUND_CLOSEST(x, divisor)(			\
> >  {							\
> >  	typeof(x) __x = x;				\
> >  	typeof(divisor) __d = divisor;			\
> > -	(((typeof(x))-1) > 0 || (__x) > 0) ?		\
> > +	(((typeof(x))-1) > 0 ||				\
> > +	 ((typeof(divisor))-1) > 0 || (__x) > 0) ?	\
> >  		(((__x) + ((__d) / 2)) / (__d)) :	\
> >  		(((__x) - ((__d) / 2)) / (__d));	\
> >  }							\
> 
> Looks good.
> 
> -- 
> Jean Delvare
> 

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Jean Delvare]

Hi Guenter,

On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> > The changelog didn't describe the end-user visible effects of the bug. 
> > Please always include this information.  Because...
>
> One observed effect is that the s2c_hwmon driver reports a value of 4198403
> instead of 0 if the ADC reads 0.
> 
> Other impact is unpredictable. Problem is seen if the divisor is an unsigned
> variable or constant and the dividend is less than (divisor/2).

Really? In my own testing, the problem only shows with dividend == 0,
and even then, only when dividend is signed and divisor is not.
DIV_ROUND_CLOSEST(5, 20U) returns 0 as expected, and so do
DIV_ROUND_CLOSEST(0 / 20), DIV_ROUND_CLOSEST(0U / 20) and
DIV_ROUND_CLOSEST(0U / 20U).

Are your observations different?

> > The patch is applicable to 3.7.x.  Should we backport it?
>
> Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
> unpredictable.
> 
> 3.6 needs it as well.

-- 
Jean Delvare

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Juergen Beisert]

Hi Jean,

Jean Delvare wrote:
> On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> > On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> > > The changelog didn't describe the end-user visible effects of the bug.
> > > Please always include this information.  Because...
> >
> > One observed effect is that the s2c_hwmon driver reports a value of
> > 4198403 instead of 0 if the ADC reads 0.
> >
> > Other impact is unpredictable. Problem is seen if the divisor is an
> > unsigned variable or constant and the dividend is less than (divisor/2).
>
> Really? In my own testing, the problem only shows with dividend == 0,
> and even then, only when dividend is signed and divisor is not.
> DIV_ROUND_CLOSEST(5, 20U) returns 0 as expected, and so do
> DIV_ROUND_CLOSEST(0 / 20), DIV_ROUND_CLOSEST(0U / 20) and
> DIV_ROUND_CLOSEST(0U / 20U).
>
> Are your observations different?

I tried it with this simple user-land program to get an idea what's going 
wrong in the s3c_hwmon.c ADC driver:

#define DIV_ROUND_CLOSEST(x, divisor)(			\
{							\
	typeof(x) __x = x;				\
	typeof(divisor) __d = divisor;			\
	(((typeof(x))-1) > 0 || (__x) > 0) ?		\
		(((__x) + ((__d) / 2)) / (__d)) :	\
		(((__x) - ((__d) / 2)) / (__d));	\
}							\
)

int main(int argc, char *argv[])
{
	int x;
	unsigned y;

	printf("Constants\n");

	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(-1, 2));
	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(-1, 1023));
	printf("0 -> %d\n", DIV_ROUND_CLOSEST(0, 1023));
	printf("0 -> %d\n", DIV_ROUND_CLOSEST(0, 2));
	printf("1 -> %d\n", DIV_ROUND_CLOSEST(1, 2));
	printf("1 -> %d\n", DIV_ROUND_CLOSEST(3300, 1023));
	printf("2 -> %d\n", DIV_ROUND_CLOSEST(6600, 1023));

	printf("Variables\n");

	x = -1; y = 2;
	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(x, y));
	x = -1; y = 1023;
	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(x, y));
	x = 0; y = 1023;
	printf("0 -> %d\n", DIV_ROUND_CLOSEST(x, y));
	x = 3300; y = 1023;
	printf("3300 -> %d\n", DIV_ROUND_CLOSEST(3300, 1023));
	x = 6600; y = 1023;
	printf("6600 -> %d\n", DIV_ROUND_CLOSEST(6600, 1023));

	return 0;
}

Result is on my x86 host (same on my ARM target):

Constants
-1 -> -1
-1 -> 0
0 -> 0
0 -> 0
1 -> 1
1 -> 3
2 -> 6
Variables
-1 -> 2147483647
-1 -> 4198403
0 -> 4198403
3300 -> 3
6600 -> 6

Regards,
Juergen

-- 
Pengutronix e.K.                              | Juergen Beisert             |
Linux Solutions for Science and Industry      | http://www.pengutronix.de/  |

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Jean Delvare]

Hi Juergen,

On Thu, 20 Dec 2012 11:30:38 +0100, Juergen Beisert wrote:
> Hi Jean,
> 
> Jean Delvare wrote:
> > On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> > > One observed effect is that the s2c_hwmon driver reports a value of
> > > 4198403 instead of 0 if the ADC reads 0.
> > >
> > > Other impact is unpredictable. Problem is seen if the divisor is an
> > > unsigned variable or constant and the dividend is less than (divisor/2).
> >
> > Really? In my own testing, the problem only shows with dividend == 0,
> > and even then, only when dividend is signed and divisor is not.
> > DIV_ROUND_CLOSEST(5, 20U) returns 0 as expected, and so do
> > DIV_ROUND_CLOSEST(0 / 20), DIV_ROUND_CLOSEST(0U / 20) and
> > DIV_ROUND_CLOSEST(0U / 20U).
> >
> > Are your observations different?
> 
> I tried it with this simple user-land program to get an idea what's going 
> wrong in the s3c_hwmon.c ADC driver:
> 
> #define DIV_ROUND_CLOSEST(x, divisor)(			\
> {							\
> 	typeof(x) __x = x;				\
> 	typeof(divisor) __d = divisor;			\
> 	(((typeof(x))-1) > 0 || (__x) > 0) ?		\
> 		(((__x) + ((__d) / 2)) / (__d)) :	\
> 		(((__x) - ((__d) / 2)) / (__d));	\
> }							\
> )
> 
> int main(int argc, char *argv[])
> {
> 	int x;
> 	unsigned y;
> 
> 	printf("Constants\n");
> 
> 	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(-1, 2));
> 	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(-1, 1023));
> 	printf("0 -> %d\n", DIV_ROUND_CLOSEST(0, 1023));
> 	printf("0 -> %d\n", DIV_ROUND_CLOSEST(0, 2));
> 	printf("1 -> %d\n", DIV_ROUND_CLOSEST(1, 2));
> 	printf("1 -> %d\n", DIV_ROUND_CLOSEST(3300, 1023));
> 	printf("2 -> %d\n", DIV_ROUND_CLOSEST(6600, 1023));

This all works properly, because everything is signed here.
 
> 	printf("Variables\n");
> 
> 	x = -1; y = 2;
> 	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(x, y));
> 	x = -1; y = 1023;
> 	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(x, y));
> 	x = 0; y = 1023;
> 	printf("0 -> %d\n", DIV_ROUND_CLOSEST(x, y));
> 	x = 3300; y = 1023;
> 	printf("3300 -> %d\n", DIV_ROUND_CLOSEST(3300, 1023));
> 	x = 6600; y = 1023;
> 	printf("6600 -> %d\n", DIV_ROUND_CLOSEST(6600, 1023));

I don't think variables vs. constants make any difference. What makes a
difference is signed vs. unsigned. You see failures here because y is
unsigned. You'd see the same with the constants above by changing 2 to
2U and 1023 to 1023U.

> 
> 	return 0;
> }
> 
> Result is on my x86 host (same on my ARM target):
> 
> Constants
> -1 -> -1
> -1 -> 0
> 0 -> 0
> 0 -> 0
> 1 -> 1
> 1 -> 3
> 2 -> 6
> Variables
> -1 -> 2147483647
> -1 -> 4198403
> 0 -> 4198403
> 3300 -> 3
> 6600 -> 6

I see the same here with your test program.

-- 
Jean Delvare

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Jean Delvare]

On Wed, 19 Dec 2012 15:01:44 -0800, Guenter Roeck wrote:
> On Wed, Dec 19, 2012 at 11:21:15PM +0100, Jean Delvare wrote:
> > Hi Guenter,
> > 
> > On Wed, 19 Dec 2012 06:40:15 -0800, Guenter Roeck wrote:
> > > Commit 263a523 fixes a warning seen with W=1 due to change in
> > > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> > > with unsigned divisors to unsigned, even if the dividend is signed and
> > > negative (for example, -10 / 5U = 858993457). The C standard says "If one
> > > operand has unsigned int type, the other operand is converted to unsigned
> > > int", so the compiler is not to blame.
> > 
> > This is surprising to say the least. But if the C standard says so...
>
> Agreed, but it is how it is.
> 
> > I wouldn't be surprised if there are bugs because of this in the kernel
> > and in other projects.
>
> Might easily be. This might make a good interview question - I suspect many
> if not most engineers would fail it. At least I would have until yesterday :).

Neither did I. And I'm not sure I'll remember it in one year from now.

> > (...)
> > Thinking a bit more about this... Documenting the non-working cases is
> > great, however I don't really expect all developers to pay attention. I
> > can also imagine variable types changing from signed to unsigned later,
> > and never thinking this can introduce a bug.
> > 
> > So, is there nothing we can do to spot at least the second issue at
> > build time? For regular division there's nothing we can do (although I
> > don't understand why gcc doesn't warn...) but here we get the
> > opportunity to report the issue, let's take it.
> > 
> > And given that the divisor is almost always a constant,
> > maybe we can check for negative divisors too, this would be safer and
> > the code size increase would probably be very small in practice.
> > Opinions?
>
> Agreed, though we should fix the problem now and think about reporting
> afterwards.

Yes, that's a good plan.

-- 
Jean Delvare

Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

[Guenter Roeck]

On Thu, Dec 20, 2012 at 11:22:02AM +0100, Jean Delvare wrote:
> Hi Guenter,
> 
> On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> > On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> > > The changelog didn't describe the end-user visible effects of the bug.
> > > Please always include this information.  Because...
> >
> > One observed effect is that the s2c_hwmon driver reports a value of 4198403
> > instead of 0 if the ADC reads 0.
> > 
> > Other impact is unpredictable. Problem is seen if the divisor is an unsigned
> > variable or constant and the dividend is less than (divisor/2).
> 
> Really? In my own testing, the problem only shows with dividend == 0, and even
> then, only when dividend is signed and divisor is not.  DIV_ROUND_CLOSEST(5,
> 20U) returns 0 as expected, and so do DIV_ROUND_CLOSEST(0 / 20),
> DIV_ROUND_CLOSEST(0U / 20) and DIV_ROUND_CLOSEST(0U / 20U).
> 
> Are your observations different?
> 

Hmm, you are right - it only happens with 0. I thought I had also seen it with
other values.

> > > The patch is applicable to 3.7.x.  Should we backport it?
> >
> > Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
> > unpredictable.
> > 
> > 3.6 needs it as well.
> 
Turns out 3.6 is EOL, so we'll only need it in 3.7.

Guenter