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

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Message-ID: <20121026130408.GH2778@yliu-dev.sh.intel.com>
Date:	Fri, 26 Oct 2012 21:04:08 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Stefani Seibold <stefani@...bold.net>
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

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?

Thanks,
Yuanhan Liu


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