netdev - Re: [Ksummit-2010-discuss] [v2] Remaining BKL users, what to do

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Date:	Tue, 19 Oct 2010 22:54:46 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Arnd Bergmann <arnd@...db.de>, Dave Airlie <airlied@...il.com>,
	Theodore Kilgore <kilgota@...ach.math.auburn.edu>,
	Greg KH <greg@...ah.com>, codalist@...emann.coda.cs.cmu.edu,
	autofs@...ux.kernel.org, Samuel Ortiz <samuel@...tiz.org>,
	Jan Kara <jack@...e.cz>,
	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	Jan Harkes <jaharkes@...cmu.edu>, netdev@...r.kernel.org,
	Anders Larsen <al@...rsen.net>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	Bryan Schumaker <bjschuma@...app.com>,
	Christoph Hellwig <hch@...radead.org>,
	ksummit-2010-discuss@...ts.linux-foundation.org,
	Petr Vandrovec <vandrove@...cvut.cz>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-fsdevel@...r.kernel.org,
	Evgeniy Dushistov <dushistov@...l.ru>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Hendry <andrew.hendry@...il.com>,
	linux-media@...r.kernel.org
Subject: Re: [Ksummit-2010-discuss] [v2] Remaining BKL users, what to do

On Tue, Oct 19, 2010 at 08:39:58AM -0400, Steven Rostedt wrote:
> On Tue, 2010-10-19 at 09:26 +0200, Arnd Bergmann wrote:
> > On Tuesday 19 October 2010 06:52:32 Dave Airlie wrote:
> > > > I might be able to find some hardware still lying around here that uses an
> > > > i810. Not sure unless I go hunting it. But I get the impression that if
> > > > the kernel is a single-CPU kernel there is not any problem anyway? Don't
> > > > distros offer a non-smp kernel as an installation option in case the user
> > > > needs it? So in reality how big a problem is this?
> > > 
> > > Not anymore, which is my old point of making a fuss. Nowadays in the
> > > modern distro world, we supply a single kernel that can at runtime
> > > decide if its running on SMP or UP and rewrite the text section
> > > appropriately with locks etc. Its like magic, and something like
> > > marking drivers as BROKEN_ON_SMP at compile time is really wrong when
> > > what you want now is a runtime warning if someone tries to hotplug a
> > > CPU with a known iffy driver loaded or if someone tries to load the
> > > driver when we are already in SMP mode.
> > 
> > We could make the driver run-time non-SMP by adding
> > 
> > 	if (num_present_cpus() > 1) {
> > 		pr_err("i810 no longer supports SMP\n");
> > 		return -EINVAL;
> > 	}
> > 
> > to the init function. That would cover the vast majority of the
> > users of i810 hardware, I guess.
> 
> I think we also need to cover the PREEMPT case too. But that could be a
> compile time check, since you can't boot a preempt kernel and make it
> non preempt.
> 
There are enough nameless embedded vendors that have turned a preempt
kernel in to a non-preempt one at run-time by leaking the preempt count,
whether by design or not, so it's certainly possile :-)
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