linux-kernel - Re: [announce] "kill the Big Kernel Lock (BKL)" tree

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Date:	Wed, 14 May 2008 14:45:32 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexander Viro <viro@....linux.org.uk>
Subject: Re: [announce] "kill the Big Kernel Lock (BKL)" tree

On Wed, 14 May 2008, Alan Cox wrote:
> 
> 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.

The good news is that I suspect they are going away. It probably is mainly 
tty and /proc by now, and /proc is pretty close to done.

It's hard to have too many inter-module dependencies when most of the core 
modules no longer even take the kernel lock any more.

In the VFS layer, we still have 

 - the ioctl thing, obviously. That's just mind-numbing "move things 
   down", not hard per se. But there's a *lot* of them (and I suspect the 
   huge majority of them don't actually need it, since they'd already be 
   racing against read/write anyway if they did).

 - default_llseek(). Probably the same, just a lot less of it.

 - superblock read/write.

and the latter one in particular is really dubious (we already have 
"[un]lock_super()" around it all, I think).

The core kernel, VM and networking already don't really do BKL. And it's 
seldom the case that subsystems interact with other unrelated subsystems 
outside of the core areas.

So it's a lot of work, no doubt, but I do think we should be able to do 
it. The most mind-numbing part is literally all the ioctl crud. There's 
more ioctl points than there are lock_kernel() calls left anywhere else.

		Linus
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