netdev - Re: [PATCH 03/16] dma-debug: add hash functions for dma_debug

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Date:	Wed, 14 Jan 2009 18:48:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joerg Roedel <joerg.roedel@....com>, linux-kernel@...r.kernel.org,
	mingo@...hat.com, dwmw2@...radead.org,
	fujita.tomonori@....ntt.co.jp, netdev@...r.kernel.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 03/16] dma-debug: add hash functions for
	dma_debug_entries


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 14 Jan 2009 12:43:47 +0100 Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > On Fri, 9 Jan 2009 17:19:17 +0100 Joerg Roedel <joerg.roedel@....com> wrote:
> > > 
> > > > +struct hash_bucket {
> > > > +	struct list_head list;
> > > > +	spinlock_t lock;
> > > > +} ____cacheline_aligned;
> > > 
> > > __cacheline_aligned_in_smp.
> > > 
> > > This all looks like an exotically large amount of code for a debug 
> > > thingy?
> > 
> > this code checks the DMA usage of ~1 million lines of kernel code - all 
> > the DMA using drivers. I think Joerg's feature is hugely relevant as DMA 
> > scribbles are one of the hardest to debug kernel bugs: they can end up in 
> > permanent data corruption or other hard to find bugs. In fact i think his 
> > patchset is rather simple and even having 10 times as much debug code 
> > would pay for its existence in the long run.
> > 
> 
> Have we previously found bugs by other means which this facility would 
> have detected?  I don't recall any...

btw., during the past decade we have had countless very ugly driver DMA 
bugs in the past that took vendors months and specialized equipment to 
track down.

I cannot give you a number breakdown, only an impression: storage drivers 
tended to be the hardest hit (due to the severity of the bugs and due to 
their inherent complexity) - but DMA bugs in networking drivers can be 
hard to track down too.

Plus there's another benefit: if a driver passes this checking and there's 
still DMA related corruption observed, then the hardware / firmware 
becomes a stronger suspect. This helps debugging too.

	Ingo
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