linux-kernel - Re: [PATCH] sound_core.c: Remove BKL from soundcore

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Message-ID: <20091011111543.1cbb9a0e@tpl.lwn.net>
Date:	Sun, 11 Oct 2009 11:15:43 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Jonathan Corbet <corbet@....net>
Cc:	John Kacur <jkacur@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vincent^M^J Sanders <vince@...tec.co.uk>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sound_core.c: Remove BKL from soundcore_open

On Sun, 11 Oct 2009 09:20:15 -0600
Jonathan Corbet <corbet@....net> wrote:

> Changing the
> BKL to a mutex is a real semantic change which requires a real survey
> of the code affected.

One other aspect of this I forgot to mention...it's actually possible
(if unlikely) that one of those lower-level open routines depends on
the BKL's release-on-sleep semantics.  Swapping in a mutex would change
that behavior, possibly resulting in deadlocks.

I think it was Alan who once pointed out that the BKL is badly
misnamed.  It isn't really a lock, it's a modified execution
environment designed to let naive kernel code think it's still running
in a uniprocessor, no-preemption situation.  Replacing the BKL with a
different lock changes that environment, so one has to be *really*
careful about looking for any assumptions which may remain in the code.

That's why BKL-hunting is harder than it looks - and why the BKL has
hung around for all these years.

jon
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