Message-ID: <20121007014447.GB2485@linux.vnet.ibm.com>
Date:	Sat, 6 Oct 2012 18:44:47 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	Kees Cook <keescook@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] make CONFIG_EXPERIMENTAL invisible and default

On Sat, Oct 06, 2012 at 06:10:36PM +0200, Frederic Weisbecker wrote:
> 2012/10/5 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > 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.)
> 
> Yeah neither am I. With a dependency on CONFIG_BROKEN, it considerably
> reduce the testing coverage too.

;-)

> > 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?
> 
> Really I would much prefer to add some "Don't enable it unless you're
> doing kernel hacking.
> If unsure say N" text in the Kconfig.
> 
> I can understand that distros want to cover as much feature as they
> can for their users. But
> should it be an excuse for not reading outstanding warnings in Kconfig
> help text?

In my experience, they do not read these warnings carefully.  :-(
Or perhaps they do read them, but react to them by running the code
through some test suite rather than by putting full faith in the
warning.

> Or may be add some specific warning yeah. I wouldn't mind much.

We have some weeks to think about it -- I cannot see pushing a
warning in as a regression.  ;-)

							Thanx, Paul

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