linux-kernel - Re: [PATCH 1/5] kfifo: remove unnecessary type check

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Message-ID: <20130110071223.GE304@yliu-dev.sh.intel.com>
Date:	Thu, 10 Jan 2013 15:12:23 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/5] kfifo: remove unnecessary type check

On Wed, Jan 09, 2013 at 04:29:39PM +0100, Stefani Seibold wrote:
> Am Mittwoch, den 09.01.2013, 10:35 +0800 schrieb Yuanhan Liu:
> > On Tue, Jan 08, 2013 at 10:51:04PM +0100, Stefani Seibold wrote:
> > > Am Dienstag, den 08.01.2013, 22:57 +0800 schrieb Yuanhan Liu:
> > > > Firstly, this kind of type check doesn't work. It does something similar
> > > > as 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.
> > > > 
> > > 
> > > Thats wrong.
> > > 
> > > First the type checking will be used in kfifo_put() and kfifo_in() for
> > > const types to check if the passed type of the data can converted to the
> > > fifo element type. 
> > 
> > Hi Stefani,
> > 
> > Yes, I see now. After rechecking the code, I found that this kind of
> > type checking only works for those static defined kifo by
> > DECLARE/DEFINE_KFIFO. As the ptrtype is the same as the data type:
> > 
> >     /* the 4th argument "type" is "ptrtype" */
> >     #define STRUCT_KFIFO(type, size) struct __STRUCT_KFIFO(type, size, 0, type)
> > 
> >     #define DECLARE_KFIFO(fifo, type, size) STRUCT_KFIFO(type, size) fifo
> > 
> > While, for those kfifo dynamically allocated, the type checking will not
> > work as expected then as ptrtype is always "void":
> > 
> >     struct kfifo __STRUCT_KFIFO_PTR(unsigned char, 0, void);
> > 
> 
> You should avoid using struct kfifo, as you can read in kfifo.h this is
> only for compatibility reason.

Well, the fact is struct kfifo is used far more widely than
DECLARE/DEFINE_KFIFO; say above 50 vs less than 10.

Thanks.

	--yliu
> 
> If you use the macro DECLARE_KFIFO_PTR(), DECLARE_KFIFO() or
> DEFINE_KFIFO() instead.
> 
> Have a look at the examples files in the samples/kfifo directory.
> 
> - Stefani
> 
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