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SubjectRe: [PATCH 03/16] dma-debug: add hash functions for dma_debug_entries

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

> On Wed, 14 Jan 2009 12:43:47 +0100 Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > * Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > > On Fri, 9 Jan 2009 17:19:17 +0100 Joerg Roedel <joerg.roedel@amd.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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