From: Olaf Kirch <okir@suse.de>
Subject: Re: file system read locks
Date: Fri, 20 Aug 2004 17:39:21 +0200
Sender: nfs-admin@lists.sourceforge.net
Message-ID: <20040820153921.GB6861@suse.de>
References: <Pine.LNX.4.60.0408200907020.6148@harp.ngdc.noaa.gov>
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Cc: nfs@lists.sourceforge.net
To: "Ara.T.Howard" <Ara.T.Howard@noaa.gov>
In-Reply-To: <Pine.LNX.4.60.0408200907020.6148@harp.ngdc.noaa.gov>
Errors-To: nfs-admin@lists.sourceforge.net

On Fri, Aug 20, 2004 at 09:08:15AM -0600, Ara.T.Howard wrote:
> i have a perfectly functioning filesystem based write lock algorithim
> (link(2)).

Except that these FS based approaches don't support blocking; you
always have to poll.

> has anyone out there come up with an algorithim to make __read__
> locks using file system primitives?

Take a directory X. If the directory exists and is empty, the lock is not
taken by anyone. To take a read lock, create a file in that directory.
To take a write lock, remove the directory.

(This scheme has the drawback that it's highly unfair to writers, but you
can probably make it favor writers if you start to move it around rather
than rmdir it)

Olaf
-- 
Olaf Kirch     |  The Hardware Gods hate me.
okir@suse.de   |
---------------+ 


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