+ fix-mult_frac-multiple-argument-evaluation-bug.patch added to mm-nonmm-unstable branch

+ fix-mult_frac-multiple-argument-evaluation-bug.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: include/linux/math.h: fix mult_frac() multiple argument evaluation bug
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fix-mult_frac-multiple-argument-evaluation-bug.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fix-mult_frac-multiple-argument-evaluation-bug.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Alexey Dobriyan <adobriyan@gmail.com>
Subject: include/linux/math.h: fix mult_frac() multiple argument evaluation bug
Date: Sat, 20 May 2023 21:25:19 +0300

mult_frac() evaluates _all_ arguments multiple times in the body.

Clarify comment while I'm at it.

Link: https://lkml.kernel.org/r/f9f9fdbb-ec8e-4f5e-a998-2a58627a1a43@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/math.h |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

--- a/include/linux/math.h~fix-mult_frac-multiple-argument-evaluation-bug
+++ a/include/linux/math.h
@@ -118,17 +118,17 @@ __STRUCT_FRACT(s32)
 __STRUCT_FRACT(u32)
 #undef __STRUCT_FRACT
 
-/*
- * Multiplies an integer by a fraction, while avoiding unnecessary
- * overflow or loss of precision.
- */
-#define mult_frac(x, numer, denom)(			\
-{							\
-	typeof(x) quot = (x) / (denom);			\
-	typeof(x) rem  = (x) % (denom);			\
-	(quot * (numer)) + ((rem * (numer)) / (denom));	\
-}							\
-)
+/* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
+#define mult_frac(x, n, d)	\
+({				\
+	typeof(x) x_ = (x);	\
+	typeof(n) n_ = (n);	\
+	typeof(d) d_ = (d);	\
+				\
+	typeof(x_) q = x_ / d_;	\
+	typeof(x_) r = x_ % d_;	\
+	q * n_ + r * n_ / d_;	\
+})
 
 #define sector_div(a, b) do_div(a, b)
 
_

Patches currently in -mm which might be from adobriyan@gmail.com are

add-intptr_t.patch
fix-mult_frac-multiple-argument-evaluation-bug.patch

Re: + fix-mult_frac-multiple-argument-evaluation-bug.patch added to mm-nonmm-unstable branch

[Thomas Gleixner]

On Mon, May 22 2023 at 14:15, Andrew Morton wrote:
> ------------------------------------------------------
> From: Alexey Dobriyan <adobriyan@gmail.com>
> Subject: include/linux/math.h: fix mult_frac() multiple argument evaluation bug
> Date: Sat, 20 May 2023 21:25:19 +0300
>
> mult_frac() evaluates _all_ arguments multiple times in the body.

I'm not opposed to the patch, but to the description.

Multiple evaluation is not a bug per se.

Unless there is a reasonable explanation for the alleged bug this is
just a cosmetic exercise.

Changelogs have to be self explanatory and if the shortlog, aka
$subject, claims "bug" then there has to be a reasonable explanation
what the actual bug is.

Seriously.

All this is documented, but obviously documention for changelogs and the
acceptance of patches is just there to be ignored, right?

Thanks,

        tglx

Re: + fix-mult_frac-multiple-argument-evaluation-bug.patch added to mm-nonmm-unstable branch

[Alexey Dobriyan]

On Tue, May 23, 2023 at 01:45:40AM +0200, Thomas Gleixner wrote:
> On Mon, May 22 2023 at 14:15, Andrew Morton wrote:
> > ------------------------------------------------------
> > From: Alexey Dobriyan <adobriyan@gmail.com>
> > Subject: include/linux/math.h: fix mult_frac() multiple argument evaluation bug
> > Date: Sat, 20 May 2023 21:25:19 +0300
> >
> > mult_frac() evaluates _all_ arguments multiple times in the body.
> 
> I'm not opposed to the patch, but to the description.
> 
> Multiple evaluation is not a bug per se.

It is kind of a bug if a macro pretends to be a function and is spelled in
lowercase.

> Unless there is a reasonable explanation for the alleged bug this is
> just a cosmetic exercise.

Most usages looks OK, and compiler tend to merge loads so even more
usages are OK. But formally this is not OK:

	static inline unsigned long vfs_pressure_ratio(unsigned long val)
	{
	        return mult_frac(val, sysctl_vfs_cache_pressure, 100);
	}

> Changelogs have to be self explanatory and if the shortlog, aka
> $subject, claims "bug" then there has to be a reasonable explanation
> what the actual bug is.
> 
> Seriously.
> 
> All this is documented, but obviously documention for changelogs and the
> acceptance of patches is just there to be ignored, right?

I don't want to return to kindergarten and document problem which every
C programmer learns exploring MIN(a, b).

Re: + fix-mult_frac-multiple-argument-evaluation-bug.patch added to mm-nonmm-unstable branch

[Thomas Gleixner]

On Tue, May 23 2023 at 11:48, Alexey Dobriyan wrote:
> On Tue, May 23, 2023 at 01:45:40AM +0200, Thomas Gleixner wrote:
>> Changelogs have to be self explanatory and if the shortlog, aka
>> $subject, claims "bug" then there has to be a reasonable explanation
>> what the actual bug is.
>> 
>> Seriously.
>> 
>> All this is documented, but obviously documention for changelogs and the
>> acceptance of patches is just there to be ignored, right?
>
> I don't want to return to kindergarten and document problem which every
> C programmer learns exploring MIN(a, b).

A quick summary what the bug is, is _not_ kindergarten level.

Why does a reviewer have to do his own analysis at the patch level to
figure out what this solves and fixes?

It's 20 seconds of courtesy on the submitter side which saves a lot of
time on the reviewer and maintainer side.

Thanks,

        tglx

+ fix-mult_frac-multiple-argument-evaluation-bug.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: include/linux/math.h: fix mult_frac() multiple argument evaluation bug
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fix-mult_frac-multiple-argument-evaluation-bug.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fix-mult_frac-multiple-argument-evaluation-bug.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Alexey Dobriyan <adobriyan@gmail.com>
Subject: include/linux/math.h: fix mult_frac() multiple argument evaluation bug
Date: Fri, 19 May 2023 23:24:54 +0300

mult_frac() evaluates _all_ arguments multiple times in the body.

Clarify comment while I'm at it.

Link: https://lkml.kernel.org/r/f522ad25-f899-4526-abc4-da35868b6a8b@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/math.h |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

--- a/include/linux/math.h~fix-mult_frac-multiple-argument-evaluation-bug
+++ a/include/linux/math.h
@@ -118,17 +118,17 @@ __STRUCT_FRACT(s32)
 __STRUCT_FRACT(u32)
 #undef __STRUCT_FRACT
 
-/*
- * Multiplies an integer by a fraction, while avoiding unnecessary
- * overflow or loss of precision.
- */
-#define mult_frac(x, numer, denom)(			\
-{							\
-	typeof(x) quot = (x) / (denom);			\
-	typeof(x) rem  = (x) % (denom);			\
-	(quot * (numer)) + ((rem * (numer)) / (denom));	\
-}							\
-)
+/* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
+#define mult_frac(x, n, d)	\
+({				\
+	typeof(x) x_ = (x);	\
+	typeof(n) n_ = (n);	\
+	typeof(d) d_ = (d);	\
+				\
+	typeof(x) q = x_ / d_;	\
+	typeof(x) r = x_ % d_;	\
+	q * n_ + r * n_ / d_;	\
+})
 
 #define sector_div(a, b) do_div(a, b)
 
_

Patches currently in -mm which might be from adobriyan@gmail.com are

add-intptr_t.patch
fix-mult_frac-multiple-argument-evaluation-bug.patch