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Date: Wed, 14 May 2008 22:19:57 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Thomas Gleixner <tglx@linutronix.de>,
       Alexander Viro <viro@ftp.linux.org.uk>
Subject: Re: [announce] "kill the Big Kernel Lock (BKL)" tree
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> Most?

Yes

> 
> >of the legacy users inflict that locking on other code - eg the ISN
> > use of the BKL directly impacts on the tty layer work.
> 
> So you just stick unlock_kernel()/lock_kernel() around the call
> to TTY (or similar to the entry points)

It isn't that simple - I've spent a good deal of time working on it.
There are lots of paths that rely on interactions between modules. Eg we
found stuff racing between the pid structs tty internals and procfs that
happened to be saved by the BKL.

That in itself is a problem Ingo's stuff won't help with: We have lots of
"magic" accidental, undocumented and pot luck BKL locking semantics
between subsystems that are not even visible.