linux-kernel - Re: [PATCH 00/13] UAPI header file split

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Message-ID: <20120803001530.GC8250@linux.vnet.ibm.com>
Date:	Thu, 2 Aug 2012 17:15:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	arnd@...db.de, tglx@...utronix.de, mingo@...nel.org,
	davej@...hat.com
Subject: Re: [PATCH 00/13] UAPI header file split

On Fri, Jul 20, 2012 at 10:56:37PM +0100, David Howells wrote:
> 
> Here's the second installment of patches from step 1 of my plan below to clean
> up the kernel header files and sort out the inclusion recursion problems.
> 
> Note that these patches will need regenerating if the header files they alter
> change before they're applied.  However, the disintegration is scripted, so
> that just takes a few minutes normally.
> 
> 
> ===================================
> BACKGROUND ON THE RECURSION PROBLEM
> ===================================
> 
> I occasionally run into a problem where I can't write an inline function in a
> header file because I need to access something from another header that
> includes this one.  Due to this, I end up writing it as a #define instead.
> 
> The problems are mainly due to inline functions.  If we split some headers
> (linux/sched.h being the biggest culprit) to separate the inline functions from
> the data structs (e.g. task_struct) then we could reduce the problems.  Other
> splits and rearrangements could help also.
> 
> Quite often it's a case of an inline function in header A wanting a struct[*]
> from header B, but header B already has an inline function that wants a struct
> from header A.
> 
> 	[*] or constant or whatever.
> 
> In the past someone tried to add a kernel-offsets file (an analogue to
> asm-offsets) to deal with the problems of dealing with both linux/rcupdate.h
> and linux/sched.h - each header needed to be included before the other.

Indeed, and later attempts to work around this problem using per-CPU
variables did not go well either.  This separation should allow
__rcu_read_lock() to be inlined for PREEMPT=y kernels, which would be
quite nice.

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

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