netdev - Re: [PATCH] igb: completely disable interrupts during netpoll

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Message-ID: <20080626181252.GD32408@gospo.rdu.redhat.com>
Date:	Thu, 26 Jun 2008 14:12:52 -0400
From:	Andy Gospodarek <andy@...yhouse.net>
To:	David Miller <davem@...emloft.net>
Cc:	jesse.brandeburg@...el.com, andy@...yhouse.net,
	jeffrey.t.kirsher@...el.com, bruce.w.allan@...el.com,
	john.ronciak@...el.com, netdev@...r.kernel.org
Subject: Re: [PATCH] igb: completely disable interrupts during netpoll

On Thu, Jun 26, 2008 at 02:16:58AM -0700, David Miller wrote:
> From: "Brandeburg, Jesse" <jesse.brandeburg@...el.com>
> Date: Thu, 26 Jun 2008 00:23:59 -0700
> 
> > I think the deadlocks occur (only when using netpoll) because the tx
> > cleanup (igb_clean_tx_irq) is being called simultaneously from two
> > routines, which generally is guaranteed not to happen in regular
> > everyday napi mode because of the bh_disable before calling dev->poll.
> 
> Thanks for the info, it's clear what the IGB driver bug is.
> 
> It's that the TX reclaim is not happening in the ->poll() handler.  If
> you did that, all of this would work out correctly and you wouldn't
> need to disable the IRQ with disable_irq() to prevent deadlocks.
> 
> Instead IGB what does is do TX reclaim inside of it's hw interrupt
> handler.
> 
> And that is actually making much more work than if you did it properly
> from a NAPI ->poll() context.  When igb_unmap_and_free_tx_resource()
> runs from the HW interrupt it is going to queue these packet frees up
> into the mid-layer's per-cpu TX free queue, and that will schedule yet
> another software interrupt to actually run the kfree_skb()'s on all of
> these liberated TX frames.
> 
> I moved the napi context out of the generic net device, use it! :-)
> And then you can call dev_kfree_skb() directly from
> igb_unmap_and_free_tx_resource() instead of dev_kfree_skb_any().

This is basically the same problem that I fixed with e1000 and posted
last week.  I remember that igb and ixgbe had the same possible, bug but
hadn't gotten around to posting it yet.

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