linux-kernel - Re: [patch 1/3] net: serialize hrtimer callback in sched

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Date:	Tue, 14 Jul 2009 10:55:14 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Miller <davem@...emloft.net>
cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	kaber@...sh.net, peterz@...radead.org
Subject: Re: [patch 1/3] net: serialize hrtimer callback in sched_cbq

David,

On Sun, 12 Jul 2009, David Miller wrote:

> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Thu, 09 Jul 2009 21:59:22 -0000
> 
> > The hrtimer callback cbq_undelay() is not serialized against
> > cbq_ovl_delay(). That affects at least q->pmask and q->delay_timer.
> > 
> > Lock it proper.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> The problems here are even much deeper than it appears.
> 
> First of all, I am to understand that hrtimers run from hardware
> interrupt context, right?  If so, all of these datastructures are
> softirq safe only.
> 
> And it is not merely the immediate things you see being modified in
> this hrtimer, such as ->pmask etc., it is also the q->active[]
> pointers, the list state for the classes, just about everything in the
> qdisc state is referenced in this hrtimer code path.

That's what I was worried about.
 
> I wonder how many queer unexplainable bugs we see because of this.
> 
> What should probably happen is that the hrtimer merely fires off work
> at software interrupt context (perhaps a tasklet or similar), and that
> software interrupt code take the qdisc's root lock throughout it's
> execution.

Sigh, I almost expected that the removal of the callback modes will
fire back some day.

Thanks,

	tglx

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