netdev - Re: e1000_netpoll(): disable_irq() triggers might

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Date:	Wed, 29 Oct 2014 21:07:13 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Sabrina Dubroca <sd@...asysnail.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, jeffrey.t.kirsher@...el.com
Subject: Re: e1000_netpoll(): disable_irq() triggers might_sleep() on
 linux-next

On Wed, 29 Oct 2014, Peter Zijlstra wrote:

> On Wed, Oct 29, 2014 at 08:49:03PM +0100, Thomas Gleixner wrote:
> > On Wed, 29 Oct 2014, Peter Zijlstra wrote:
> > 
> > > On Wed, Oct 29, 2014 at 07:33:00PM +0100, Thomas Gleixner wrote:
> > > > Yuck. No. You are just papering over the problem.
> > > > 
> > > > What happens if you add 'threadirqs' to the kernel command line? Or if
> > > > the interrupt line is shared with a real threaded interrupt user?
> > > > 
> > > > The proper solution is to have a poll_lock for e1000 which serializes
> > > > the hardware interrupt against netpoll instead of using
> > > > disable/enable_irq().
> > > > 
> > > > In fact that's less expensive than the disable/enable_irq() dance and
> > > > the chance of contention is pretty low. If done right it will be a
> > > > NOOP for the CONFIG_NET_POLL_CONTROLLER=n case.
> > > > 
> > > 
> > > OK a little something like so then I suppose.. But I suspect most all
> > > the network drivers will need this and maybe more, disable_irq() is a
> > > popular little thing and we 'just' changed semantics on them.
> > 
> > We changed that almost 4 years ago :) What we 'just' did was to add a
> > prominent warning into the code.
> 
> You know that is the same right... they didn't know it was broken
> therefore it wasn't :-), but now they need to go actually do stuff about
> it, an entirely different proposition.

Right, and of course the world and some more has the very same code
there:

poll_controller()
{
	disable_irq();
	dev_interrupt_handler();
	enable_irq();
}

Trying to twist my brain to come up with a solution which avoids the
spinlock, but I have a hard time to come up with one.

The only thing I came up with so far is to avoid adding locks to every
driver incarnation and instead put it into struct net_device and
provide helper functions for the lock/unlock case.

That does not change the fact that we need to deal with that on a per
driver basis :(

Thanks,

	tglx


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