linux-kernel - Re: [PATCH] kfifo: remove unnecessary type check

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Date:	Fri, 26 Oct 2012 15:42:44 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	Yuanhan Liu <yliu.null@...il.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH] kfifo: remove unnecessary type check

Am Freitag, den 26.10.2012, 21:04 +0800 schrieb Yuanhan Liu:
> On Fri, Oct 26, 2012 at 11:26:31AM +0200, Stefani Seibold wrote:
> > Am Freitag, den 26.10.2012, 15:17 +0800 schrieb Yuanhan Liu:
> > > On Fri, Oct 26, 2012 at 08:51:06AM +0200, Stefani Seibold wrote:
> > > > Am Freitag, den 26.10.2012, 14:11 +0800 schrieb Yuanhan Liu:
> > > > > On Fri, Oct 26, 2012 at 07:38:31AM +0200, Stefani Seibold wrote:
> > > > > > Am Freitag, den 26.10.2012, 09:46 +0800 schrieb Yuanhan Liu:
> > > > > > > From: Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> > > > > > > 
> > > > > > > Firstly, this kind of type check doesn't work. It does something similay
> > > > > > > like following:
> > > > > > > 	void * __dummy = NULL;
> > > > > > > 	__buf = __dummy;
> > > > > > > 
> > > > > > > __dummy is defined as void *. Thus it will not trigger warnings as
> > > > > > > expected.
> > > > > > > 
> > > > > > > Second, we don't need that kind of check. Since the prototype
> > > > > > > of __kfifo_out is:
> > > > > > > 	unsigned int __kfifo_out(struct __kfifo *fifo,  void *buf, unsigned int len)
> > > > > > > 
> > > > > > > buf is defined as void *, so we don't need do the type check. Remove it.
> > > > > > > 
> > > > > > > LINK: https://lkml.org/lkml/2012/10/25/386
> > > > > > > LINK: https://lkml.org/lkml/2012/10/25/584
> > > > > > > 
> > > > > > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > > > > > Cc: Wei Yang <weiyang@...ux.vnet.ibm.com>
> > > > > > > Cc: Stefani Seibold <stefani@...bold.net>
> > > > > > > Cc: Fengguang Wu <fengguang.wu@...el.com>
> > > > > > > Cc: Stephen Rothwell <sfr@...b.auug.org.au>
> > > > > > > Signed-off-by: Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> > > > > > > ---
> > > > 
> 
> [snip]...
> 
> > > > 
> > > > Also you have to build the kfifo samples, since this example code use
> > > > all features of the kfifo API.
> > > > 
> > > > And again: The kfifo is designed to do the many things at compile time,
> > > > not at runtime. If you modify the code, you have to check the compiler
> > > > assembler output for no degradation, especially in kfifo_put, kfifo_get,
> > > > kfifo_in, kfifo_out, __kfifo_in and __kfifo_out. Prevent runtime checks
> > > > if you can do it at compile time. This is the basic reasons to do it in
> > > > macros.
> > > 
> > > Is it enought to check kernel/kfifo.o only? I build that file with
> > > and without this patch. And then  dump it by objdump -D kernel/fifo.o to
> > > /tmp/kfifo.dump.with and /tmp/kfifo.dump.without, respectively. And the
> > > two dump file are exactly same.
> > > 
> > 
> > No, since most of the code is inlined due performace reasons, you have
> > to hack the kfifo examples output code for regressions and code
> > increase.
> 
> In my test, this patch doesn't change anything. Here are some data to
> prove that:
> 
> $ make samples/kfifo/
> $ cp samples/kfifo/*.o /tmp/before/
> 
> $ git am this-patch
> $ make samples/kfifo/
> $ cp samples/kfifo/*.o /tmp/after/
> 
> $ for i in /tmp/before/*.o; do size $i /tmp/after/`basename $i`; done
>    text    data     bss     dec     hex filename
>    1939     464     456    2859     b2b /tmp/before/bytestream-example.o
>    1939     464     456    2859     b2b /tmp/after/bytestream-example.o
>    text    data     bss     dec     hex filename
>    1423     112     296    1831     727 /tmp/before/dma-example.o
>    1423     112     296    1831     727 /tmp/after/dma-example.o
>    text    data     bss     dec     hex filename
>    1864     624     376    2864     b30 /tmp/before/inttype-example.o
>    1864     624     376    2864     b30 /tmp/after/inttype-example.o
>    text    data     bss     dec     hex filename
>    1916     464     472    2852     b24 /tmp/before/record-example.o
>    1916     464     472    2852     b24 /tmp/after/record-example.o
> # You will see that it changed nothing.
> 
> 
> $ objdump -d /tmp/before/bytestream-example.o >/tmp/bytestream-example.before
> $ objdump -d /tmp/after/bytestream-example.o >/tmp/bytestream-example.after
> $ diff /tmp/bytestream.before /tmp/bytestream.after -urN
> --- bytestream.before   2012-10-26 20:55:33.645578668 +0800
> +++ bytestream.after    2012-10-26 20:55:26.520578669 +0800
> @@ -1,5 +1,5 @@
> 
> -/tmp/bytestream-example.o:     file format elf64-x86-64
> +/tmp/bytestream-example.o:     file format elf64-x86-64
> 
> # So, as you can see, expect the filename, they are same.
> 
> 
> So, Stefani, is it what you want? Does this looks OK to you?

Perfect. It looks okay for me and i hope for you too ;-)

Acked by stefani@...bold.net

Greetings,
Stefani


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