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Re: [PATCH 24/25] r/o bind mounts: track number of mount writers

To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH 24/25] r/o bind mounts: track number of mount writers
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Mon, 24 Sep 2007 20:24:26 +0100
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Dave Hansen <haveblue@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20070924121035.8d8c6ce2.akpm@xxxxxxxxxxxxxxxxxxxx>
Mail-followup-to: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Dave Hansen <haveblue@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
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On Mon, Sep 24, 2007 at 12:10:35PM -0700, Andrew Morton wrote:
> > As we already say in various messages the percpu counters in here
> > look rather fishy.  I'd recomment to take a look at the per-cpu
> > superblock counters in XFS as they've been debugged quite well
> > now and could probably be lifted into a generic library for this
> > kind of think.  The code is mostly in fs/xfs/xfs_mount.c can
> > can be spotted by beeing under #ifdef HAVE_PERCPU_SB.
> > 
> > It also handles cases like hotplug cpu nicely that this code
> > seems to work around by always iterating over all possible cpus
> > which might not be nice on a dual core laptop with a distro kernel
> > that also has to support big iron.
> 
> hm.  How come xfs invented a new version of percpu_counters?

This code actually predates the generic percpu_counters even if it
was merged to mainline later.  Neither Dave who wrote it nor me
who reviewed it before it was merged thought of percpu_counters
probably.  Then again this code is considerably more complex due
to features actually needed in a very hot fastpath.


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