linux-kernel - Re: broken device locking, sg vs. sg

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Date:	Fri, 30 Mar 2007 14:43:27 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Eduard Bloch <edi@....de>
Cc:	linux-kernel@...r.kernel.org, debburn-devel@...ts.alioth.debian.org
Subject: Re: broken device locking, sg vs. sg_io on block devices

On Fri, Mar 30, 2007 at 01:17:44PM +0200, Eduard Bloch wrote:
> Hello,
> 
> I am talking this issue to LKML now.
> 
> Short story: using O_EXCL on /dev/srX alone does not help to prevent
> other process from killing your burn process by just reading the
> /dev/sgX device associated with yours, and vice versa. We have done the
> best we could to make safe operation (in contrary to Schilling's
> kill-this-evil-hald-thing bitching) but that is not enough, the locking
> has to be established on kernel layer.
> 
> Long story:
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=413960
> https://bugzilla.novell.com/show_bug.cgi?id=226019
> http://lists.alioth.debian.org/pipermail/debburn-devel/2007-February/000297.html
> and other error messages.
> 
> There is AFAICS no simple way to establish locking across the driver
> borders. If kernel developers have a good idea, any help is appreciated.
> 
> Below are the typical symptoms: wodim operates via /dev/sgX because the
> user chosen it this way, some other process (most likely hald) comes
> along and reads from /dev/sr0 and the drive gets confused. Boom.

You have thre problems here, and none of them are in the kernel :)

First the hardsware is broken when it can't deal with concurrent requests,
I'd try to get a refund for it.  Second wodim should never ever use
/dev/sg if the sr node is available.  And third HAL should stop poking
devices all the time.  Then again hald is a totally lost cause and
I can only recommend to uninstall it ASAP.
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