linux-kernel - Re: [GIT PULL] RCU changes for v4.12

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Message-Id: <20170502040202.GR3956@linux.vnet.ibm.com>
Date:   Mon, 1 May 2017 21:02:02 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] RCU changes for v4.12

On Mon, May 01, 2017 at 06:19:44PM -0700, Linus Torvalds wrote:
> On Mon, May 1, 2017 at 2:59 AM, Ingo Molnar <mingo@...nel.org> wrote:
> > Linus,
> >
> > Please pull the latest core-rcu-for-linus git tree from:
> >
> >    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-rcu-for-linus
> 
> I pulled this, and then after looking at it, ended up un-pulling it again.
> 
> I refuse to take that nasty <linux/rcu_segcblist.h> header file from hell.
> 
> I see absolutely no point in taking a header file of several hundred
> lines of code.
> 
> We have traditionally done too much inline code anyway, but we've
> learnt our lesson - and even back when we did too much of it, we
> didn't put random code that nobody uses and by definition cannot be
> performance-critical in big inline functions in header files.
> 
> If it was some one-liner helper function, that would be one thing. But
> there are functions that don't even fit on the screen, and that have
> multiple loops and memory barriers in them.
> 
> The one function I decided to grep for was used EXACTLY NOWHERE. Yet
> it was apparently SO INCREDIBLY important that it needed to be inlined
> in a huge header file despite being huge and complicated.
> 
> So no. This is too ugly to live, and certainly too ugly to be pulled.
> 
> The RCU code needs to start showing some good taste.
> 
> There are valid reasons to inline even large functions, if they have
> constant arguments that make us expect them to generate a single
> instruction of code in the end. But that was very much not the case
> here.
> 
> Not pulling. Try again next merge window when the code has been
> cleaned up and isn't too ugly to live.

Please accept my apologies!

I was patterning this code too much after the various *list*.h header
files, and failed to notice that the functions were getting large.
I will get rid of the unused rcu_segcblist_extract_all() function
and create a kernel/rcu/segcblist.c for the functions that are either
non-trivial or performance-insensitive.

Does that cover it, or am I missing something?

							Thanx, Paul