linux-kernel - Re: [PATCH v2 1/1] headers: fix circular dependency between linux/sched.h and linux/wait.h

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Date:	Mon, 21 Feb 2011 17:20:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Cohen <dacohen@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-omap@...r.kernel.org, linux-media@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2 1/1] headers: fix circular dependency between
 linux/sched.h and linux/wait.h

On Mon, 2011-02-21 at 18:03 +0200, David Cohen wrote:
> On Mon, Feb 21, 2011 at 5:54 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, 2011-02-21 at 16:38 +0200, David Cohen wrote:
> >> Currently sched.h and wait.h have circular dependency between both.
> >> wait.h defines macros wake_up*() which use macros TASK_* defined by
> >> sched.h. But as sched.h indirectly includes wait.h, such wait.h header
> >> file can't include sched.h too. The side effect is when some file
> >> includes wait.h and tries to use its wake_up*() macros, it's necessary
> >> to include sched.h also.
> >> This patch moves all TASK_* macros from linux/sched.h to a new header
> >> file linux/task_state.h. This way, both sched.h and wait.h can include
> >> task_state.h and fix the circular dependency. No need to include sched.h
> >> anymore when wake_up*() macros are used.
> >
> > I think Alexey already told you what you done wrong.
> >
> > Also, I really don't like the task_state.h header, it assumes a lot of
> > things it doesn't include itself and only works because its using macros
> > and not inlines at it probably should.
> 
> Like wait.h I'd say. The main issue is wait.h uses TASK_* macros but
> cannot properly include sched.h as it would create a circular
> dependency. So a file including wait.h is able to compile because the
> dependency of sched.h relies on wake_up*() macros and it's not always
> used.
> We can still drop everything else from task_state.h but the TASK_*
> macros and then the problem you just pointed out won't exist anymore.
> What do you think about it?

I'd much rather see a real cleanup.. eg. remove the need for sched.h to
include wait.h.

afaict its needed because struct signal_struct and struct sighand_struct
include a wait_queue_head_t. The inclusion seems to come through
completion.h, but afaict we don't actually need to include completion.h
because all we have is a pointer to a completion, which is perfectly
fine with an incomplete type.

This all would suggest we move the signal bits into their own header
(include/linux/signal.h already exists and seems inviting).

And then make sched.c include signal.h and completion.h.

But then, there might be a 'good' reason these signal bits live in
sched.h and not on their own, but I wouldn't know..
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