linux-kernel - Re: [GIT, RFC] Killing the Big Kernel Lock

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Date:	Wed, 24 Mar 2010 23:25:23 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Matthew Wilcox <matthew@....cx>,
	Thomas Gleixner <tglx@...utronix.de>, jblunck@...e.de,
	Alan Cox <alan@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [GIT, RFC] Killing the Big Kernel Lock

On Wednesday 24 March 2010 23:10:16 Alan Cox wrote:
> >   The basic idea here is to make recursive locking and the release-on-sleep
> >   explicit, so every mutex_lock, wait_event, workqueue_flush and schedule
> >   in the TTY layer now explicitly releases the BTM before blocking.
> 
> I'm not sure if that is actually the path of sanity (yours at least), nor
> the right way to whack the other BKL users whose use is horrible but
> essentially private.
> 
> It would be nice to get the other bits in first removing BKL from most of
> the kernel and building kernels which are non BKL except for the tty
> layer. That (after Ingo's box from hell has run it a bit) would
> reasonably test the assertion that the tty layer has no BKL requirements
> that are driven by external to tty layer code.

Yes, we can do that by applying all patches except 'tty: implement BTM
as mutex instead of BKL', which is the only one in the tty section of
my series that should really change the behaviour. Building a kernel
with all other BKL users gone currently implies disabling usbcore,
videodev, soundcore, i4l and capi, as well as a large number of obsolete
device drivers.

The only ones that I can imagine still interacting with the tty code
are the ISDN drivers, and even those look pretty unlikely.

> That to me would test the biggest question of all and be a reasonably
> good base from which to then either apply the tty BTM patches or attack
> the problem properly with the BKL localised to one subtree.

We could also make the 'tty: implement BTM as mutex instead of BKL'
patch a config option that makes it possible to test it out some more
while conservative users just continue to get the BKL semantics.

	Arnd
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