linux-kernel - Re: cdrom: use kmalloced buffers instead of buffers on stack

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Date:	Tue, 22 Apr 2008 07:48:58 +0200
From:	tsbogend@...ha.franken.de (Thomas Bogendoerfer)
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: cdrom: use kmalloced buffers instead of buffers on stack

On Mon, Apr 21, 2008 at 10:01:26PM -0400, Jeff Garzik wrote:
> >    cdrom: use kmalloced buffers instead of buffers on stack
> >    
> >    If cdrom commands are issued to a scsi drive in most cases the buffer 
> >    will be
> >    filled via dma.  This leads to bad stack corruption on non coherent 
> >    platforms,
> >    because the buffers are neither cache line aligned nor is the size a 
> >    multiple
> >    of the cache line size.  Using kmalloced buffers avoids this.
> >    
> >    Signed-off-by: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> >    Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> >---
> > drivers/cdrom/cdrom.c |  274 
> > ++++++++++++++++++++++++++++++++-----------------
> > 1 files changed, 181 insertions(+), 93 deletions(-)
> 
> Eh... AFAICS this is only really useful in two of the cases converted.
> 
> For all the other cases (<= 32 bytes), it is _far_ less complex, far 
> less code to simply communicate the additional alignment requirements to 
> the compiler.
> 
> What about __attribute__ __aligned__?  Was that tried?

I used that while narrowing down the bug. But not only the alignment is
important, but also size needs to be a multiple of the cache line size.
Which means it needs to be 128 bytes for most SGI machines. That
and the following in DMA-mapping.txt

"This rule also means that you may use neither kernel image addresses
(items in data/text/bss segments), nor module image addresses, nor
stack addresses for DMA."

let me choose the kmalloc() solution.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessary a
good idea.                                                [ RFC1925, 2.3 ]
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