From: Neil Brown <neilb@suse.de>
To: Kyle Moffett <mrmacman_g4@mac.com>
Date: Mon, 6 Feb 2006 09:27:57 +1100
Message-ID: <17382.31725.813127.10435@cse.unsw.edu.au>
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Cc: Phillip Susi <psusi@cfl.rr.com>, Jan Engelhardt <jengelh@linux01.gwdg.de>,
       Krzysztof Halasa <khc@pm.waw.pl>, Olivier Galibert <galibert@pobox.com>,
       linux-kernel@vger.kernel.org
Subject: Re: CD writing in future Linux (stirring up a hornets' nest)
In-Reply-To: message from Kyle Moffett on Sunday February 5
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On Sunday February 5, mrmacman_g4@mac.com wrote:
> 
> If you specify O_EXCL (and not O_CREAT), it is implementation defined  
> what will happen (in the Linux case, this opens a block device for  
> exclusive access).

With Linux, O_EXCL on a block devices isn't *exactly* exclusive
access.
It only provided you exclusive access against other people who ask for
exclusive access, which includes in-kernel usage like mount, md, dm,
and swap.

So if you open a block device O_EXCL, it will fail if the block device
is already open O_EXCL or is mounted, or in use by the kernel in some
other way (including if a partition is open O_EXCL).  An if you
succeed in getting an O_EXCL open, then no-one else will be able to
get an O_EXCL open, or mount the filesystem etc.
Bit on open without O_EXCL will always succeed no matter whether
someone has it O_EXCL or not.

So it is a lot like an advisory exclusive lock on the whole block
device.

NeilBrown
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