linux-kernel - Re: [PATCH 1.5/2] generic-smp: fix initial quiesent count.

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Message-Id: <1234646056.4695.10.camel@laptop>
Date:	Sat, 14 Feb 2009 22:14:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>, Jens Axboe <jens.axboe@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1.5/2] generic-smp: fix initial quiesent count.

On Sat, 2009-02-14 at 15:46 +0100, Peter Zijlstra wrote:
> first compile, then send out...
> 
> ---
> Subject: generic-smp: fix initial quiesent count.
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Sat Feb 14 15:36:07 CET 2009
> 
> If we start with a quiesent sequence count of 0, we'll match the initial stamp
> values of the cfd_data, and never make any progress.
> 
> To avoid getting stuck in this situation, start out with an increased quiesent
> sequence count.

OK, so this is not going to fix the problem in generic. Its still
possible to end up with the original issue.

We'd need a callback list to free used entries, much like regular RCU,
this quiesent sequence count's wrapping just seems too brittle.


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