Re: [PATCH] sound_core.c: Remove BKL from soundcore_open

[Jonathan Corbet]

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