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

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Date:	Tue, 14 Jul 2009 10:22:02 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	David Miller <davem@...emloft.net>
CC:	tglx@...utronix.de, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: Re: [patch 1/3] net: serialize hrtimer callback in sched_cbq

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.
> 
> 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.

That's my understanding what HRTIMER_SOFTIRQ is used for. I think
simply grabbing the root lock in cbq_undelay() should be fine.

Compile-tested only.


View attachment "01.diff" of type "text/x-patch" (1230 bytes)