netdev - Re: [raw v1 2/4] [NET] Use raw

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Message-ID: <20131008102645.GA22639@gmail.com>
Date:	Tue, 8 Oct 2013 12:26:45 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	akpm@...uxfoundation.org, Eric Dumazet <edumazet@...gle.com>,
	netdev@...r.kernel.org, Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [raw v1 2/4] [NET] Use raw_cpu ops for SNMP stats


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Oct 08, 2013 at 09:21:14AM +0200, Ingo Molnar wrote:
> > 
> > * Christoph Lameter <cl@...ux.com> wrote:
> > 
> > > SNMP stats are not protected by preemption but by bh handling.
> > 
> > Most forms of bh exclusion work via the preemption count though, and 
> > softirq contexts themselves are generally not preemptible [to other CPUs] 
> > either.
> > 
> > So the warnings should, in most cases, not trigger.
> 
> Right, so softirqs run either in the irq tail at which point 
> preempt_count += SOFTIRQ_OFFSET and thus preemption is disabled, or it 
> runs in ksoftirqd which has strict cpu affinity which also disables the 
> warning, and it also increments preempt_count with SOFTIRQ_OFFSET to 
> exclude the softirq from interrupts while its running, also disabling 
> the warning.

A third context would be syscall-level code that runs with 
local_bh_disable()/enable() - but that too ought to have the preempt count 
elevated.

> So it should very much not trigger.. if it does you want to know about 
> it.

Yes. If nothing else then for the education value.

Thanks,

	Ingo
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