linux-kernel - Re: dma-debug: check_for

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Date:	Tue, 16 Jun 2009 10:14:28 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	stefanr@...6.in-berlin.de, ben-linux@...ff.org,
	linux-kernel@...r.kernel.org
Subject: Re: dma-debug: check_for_stack background?

On Tue, Jun 16, 2009 at 03:16:37PM +0900, FUJITA Tomonori wrote:
> On Mon, 15 Jun 2009 10:13:55 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
> 
> > On Sun, Jun 14, 2009 at 11:43:42PM +0200, Stefan Richter wrote:
> > > Ben Dooks wrote:
> > >> On Sun, Jun 14, 2009 at 08:56:29PM +0200, Stefan Richter wrote:
> > >>> the DMA debug facility warns  about mappings of on-stack memory.  
> > >>> What's the problem with that?
> > > ...
> > >> I'm not sure if it is possible on all architectures, however the
> > >> DMA mapping documentation prohibts the use of stack memory.
> > >
> > > Ah, it has all been documented in plain sight for ages.  Thanks for the  
> > > pointer.
> > 
> > Problem is that the stack might go away or could be otherwise reused
> > while the DMA is in-flight. This could cause really hard to debug
> > problems.
> 
> Not true. Even if you use the stack buffer for DMA synchronously, it
> doesn't work (we had the bad code like starting dma and waiting for
> the completion).

I've not written that this is the only problem. I know that there may be
others like the one you experienced. But what I wrote could also happen
and is a problem :)

Joerg

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