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

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Date:	Sun, 1 Apr 2007 01:14:52 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
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

> > > The use of /dev/sg* is still common practice, its invention predates
> > 
> > The /dev/sg interface cannot do the locking. If you use /dev/sg you are
> 
> Again, it doesn't have to. It can pass the locking operations to the
> related block device driver.

No it can't. The driver has no idea what the locking rules are for
arbitary command blocks send to arbitary devices. /dev/sg is a *raw*
interface. You can send anything to anyone, and the locking rules for
that are far too complex for a giant morass of kernel code to get added.

The mess begins because you use /dev/sg and put it in a cdrom group
instead of using SG_IO on the /dev/sr device. The mess continues because
of the user of O_EXCL locking thus forcing re-open/close by HAL instead
of fcntl based co-operative locking.

The job of the kernel is not and never has been to anticipate and correct
everything stupid someone tries to do in user space. 

As I said before the people wanting to arbitrate serial ports got this
right in the mid 1970's your situation is not much more complicated,
unless you persist in using /dev/sg - which yes does make it hard, but so
does writing it in COBOL, or while standing on your head. And the
solution to all three cases is the same *DONT DO IT*

Alan
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