linux-kernel - Re: [rfc] "fair" rw spinlocks

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Date:	Mon, 30 Nov 2009 23:37:16 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks

On Mon, 30 Nov 2009, Linus Torvalds wrote:
> On Mon, 30 Nov 2009, Thomas Gleixner wrote:
> > 
> > Yeah, forgot to mention sched.c, but that's solvable
> 
> It should be fairly easy to add a few 'spin_lock(&tasklist_lock)' around 
> stuff that really depended on exclusion from writers. That should 
> _hopefully_ be the rare case.
> 
> The biggest problem is that there will almost inevitably be things that 
> get missed, and any races exposed by lacking locking will be _very_ hard 
> to debug and trigger. So what I'd be worried about is not getting to a 
> "practically working" state, but any really subtle cases that nobody 
> really hits in practice.

I'm aware of that. The number of places where we read_lock
tasklist_lock is 79 in 36 files right now. That's not a horrible task
to go through them one by one and do a case by case conversion with a
proper changelog. That would only leave the write_lock sites. 

We can then either do the rw_lock to spin_lock conversion or keep the
rw_lock which has no readers anymore and behaves like a spinlock for a
transition time so reverts of one of the read_lock -> rcu patches
could be done to debug stuff.

Thanks,

	tglx
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