linux-kernel - Re: [PATCH 46/50] preempt.h: Kill dependency on list.h

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Date: Sun, 17 Dec 2023 00:18:17 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, tglx@...utronix.de, x86@...nel.org,
	tj@...nel.org, peterz@...radead.org, mathieu.desnoyers@...icios.com,
	paulmck@...nel.org, keescook@...omium.org,
	dave.hansen@...ux.intel.com, mingo@...hat.com, will@...nel.org,
	longman@...hat.com, boqun.feng@...il.com, brauner@...nel.org
Subject: Re: [PATCH 46/50] preempt.h: Kill dependency on list.h

On Sat, Dec 16, 2023 at 04:04:43PM -0800, Randy Dunlap wrote:
> 
> 
> On 12/16/23 14:35, Kent Overstreet wrote:
> > On Sat, Dec 16, 2023 at 06:13:41AM +0000, Matthew Wilcox wrote:
> >> On Fri, Dec 15, 2023 at 10:35:47PM -0500, Kent Overstreet wrote:
> >>> -	INIT_HLIST_NODE(&notifier->link);
> >>> +	/* INIT_HLIST_NODE() open coded, to avoid dependency on list.h */
> >>> +	notifier->link.next = NULL;
> >>> +	notifier->link.pprev = NULL;
> >>
> >> Arguably INIT_HLIST_NODE() belongs in types.h -- we already have
> >> RCUREF_INIT() and ATOMIC_INIT() in there.
> > 
> > I think I'd prefer to keep types.h as minimal as possible - as soon as
> > we start putting non type stuff in there people won't know what the
> > distinction is and it'll grow.
> > 
> > preempt.h is a bit unusual too, normally we'd just split out a _types.h
> > header there but it's not so easy to split up usefully.
> > 
> 
> I don't feel like I have NAK power, but if I did, I would NAK
> open coding of INIT_HLIST_HEAD() or anything like it.
> I would expect some $maintainer to do likewise, but I could be
> surprised.

There is another solution here (although I prefer moving INIT_HLIST_HEAD
into types.h).  The preprocessor allows redefinitions as long as the two
definitions match exactly.  So you can copy INIT_HLIST_HEAD into
preempt.h and if the definition ever changes, we'll notice.