Date: Fri, 5 Oct 2012 09:46:42 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Kees Cook <keescook@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        "David S. Miller" <davem@davemloft.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH] make CONFIG_EXPERIMENTAL invisible and default
Message-ID: <20121005164642.GA10711@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Thu, Oct 04, 2012 at 07:31:50AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 04, 2012 at 02:55:39AM +0100, Matthew Garrett wrote:
> > On Wed, Oct 03, 2012 at 01:03:14PM -0700, Paul E. McKenney wrote:
> > 
> > > That has not proven sufficient for me in the past, RCU_FAST_NO_HZ
> > > being a case in point.
> > 
> > Taint the kernel at boot time? That'd be sufficient to force distros to 
> > disable it.
> 
> Cool!  That does sound much more socially responsible than my thought
> of forcing a splat (e.g., WARN_ON(1)) during boot.  ;-)

So, from what I can see, here is the list of the ways of warning distros
off of a given kernel config option, taken in terms of CONFIG_RCU_USER_QS:

1.	Make CONFIG_RCU_USER_QS depend on CONFIG_BROKEN.

	It sounds to me like distros would avoid adding this (do they?),
	but tester would probably avoid it as well.

2.	Make CONFIG_RCU_USER_QS depend on CONFIG_STAGING.

	As Frederic noted, this is more of a driver thing than a
	core-kernel thing, so probably not appropriate.

3.	Boot-time WARN_ON() if CONFIG_RCU_USER_QS=y.

	This seems to me to be a tad excessive.  But the place to do it
	might be rcu_bootup_announce_oddness() in kernel/rcutree_plugin.h.

4.	Remove CONFIG_RCU_USER_QS from Kconfig, so that users have to
	patch their kernel to enable it.

	This also seems a tad excessive.

5.	Maintain CONFIG_RCU_USER_QS out of tree, for example in the
	-rt patchset.

	This is a good place to start, but it has been where
	CONFIG_RCU_USER_QS has been for some time, and although it
	got some good testing, it clearly needs more.  In my view,
	CONFIG_RCU_USER_QS has outgrown its out-of-tree roots.

6.	Boot-time add_taint() if CONFIG_RCU_USER_QS=y, as suggested
	by Matthew Garrett.  The taint value might be TAINT_CRAP,
	TAINT_OOT_MODULE, TAINT_WARN, or TAINT_FIRMWARE_WORKAROUND --
	all the other taint values disable lockdep.  Of these four,
	TAINT_OOT_MODULE and TAINT_FIRMWARE_WORKAROUND are clearly
	off-topic, leaving TAINT_CRAP and TAINT_WARN.  Taking them one
	at a time:

	TAINT_CRAP: Used when loading modules from staging.

	TAINT_WARN: Used when "scheduling while atomic" is encountered.

	So I have my tongue only halfway in my cheek when I suggest
	starting with TAINT_CRAP, then moving to TAINT_WARN, then
	removing the tainting altogether.  The place to do this might
	be rcu_bootup_announce_oddness() in kernel/rcutree_plugin.h.

So how about the following progression?

A.	Early days, only a few crazies should test.  In this case, the
	code should be out of tree, perhaps in something like -rt,
	perhaps as a set of patches.

B.	Need more testers, but still not expected to work reasonably.
	Mainline, but depending on CONFIG_BROKEN.  (I am not all that
	enthusiastic about this option, but am including it for
	completeness.)

C.	Need wide testing, but don't want 100,000,000 unsuspecting
	test subjects.  Taint the kernel with TAINT_CRAP.

D.	OK for production in special situations, but definitely not
	for typical users.  Taint the kernel with TAINT_WARN.

E.	Ready for general production use.  Mainlined without restrictions.

I would say that CONFIG_RCU_USER_QS is currently at point C above, it
clearly now needs testing on a wide variety of hardware, but also is
clearly not ready for 100,000,000 users.

Thoughts?

							Thanx, Paul

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