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

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Date:	Mon, 30 Nov 2009 08:07:16 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ibm.com>
Subject: Re: [rfc] "fair" rw spinlocks



On Mon, 30 Nov 2009, Nick Piggin wrote:
> 
> Well the simple thing I tried earlier was a per-cpu array of nesting
> counter there. It's not _too_ expensive, but it does add another cacheline
> access and branch there. It seems to work in solving the livelock though.

So how did you do the nesting counter? Afaik, it needs to be something 
like

	local_irq_save(flags);
	if (!get_cpu_var(tasklist_counter)++)
		spin_lock(&tasklist_lock);
	local_irq_restore(flags);

on the read_lock side (and the same in reverse on unlock). Which seems 
quite a bit more expensive than what we have now. Especially on UP, but I 
guess you can make it conditional on CONFIG_SMP (but that won't help 
generic kernels).

				Linus
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