netdev - Re: [PATCH] macb: Add support of the netpoll API

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Date:	Fri, 17 Apr 2009 11:25:53 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	David Miller <davem@...emloft.net>
Cc:	thomas.petazzoni@...e-electrons.com, netdev@...r.kernel.org,
	michael@...e-electrons.com
Subject: Re: [PATCH] macb: Add support of the netpoll API

David Miller wrote:
> From: Haavard Skinnemoen <haavard.skinnemoen@...el.com>
> Date: Fri, 17 Apr 2009 11:07:03 +0200
> 
> > David Miller wrote:
> >> And in response to this Haavard explained the potential deadlock
> >> (sorry I deleted that email) and suggested to use local_irq_save()
> >> 
> >> But that won't work either, because it doesn't prevent the interrupt
> >> handler from running on other cpus, it only prevents that on the
> >> local cpu.
> > 
> > The interrupt handler takes bp->lock. Isn't that sufficient to prevent
> > it from running on other cpus?
> 
> We don't necessarily hold bp->lock here, we could be called from
> the netpoll code.  Another cpu could easily hold bp->lock and be
> in the middle of the interrupt handler.

Yes, but we're about to take it. poll_controller() disables interrupts
and calls the interrupt handler, which takes the lock. That's
essentially an open-coded spin_lock_irqsave().

Now, that will obviously deadlock too if we're called from within the
interrupt handler or some other place holding bp->lock...

I guess you're right -- we just have to be careful about calling
printk() from within the driver. But it seems a lot easier to just drop
the lock before dumping an error message than it is to work around that
disable_irq() deadlock. And if you're running with verbose debugging
enabled, you probably shouldn't be using netconsole anyway.

Haavard
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