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

[Dave Hansen <haveblue@us.ibm.com>]

On Mon, 2007-09-24 at 18:54 +0100, Christoph Hellwig 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.

I'll take a look at xfs to see what I can get out of it.

There are basically two times when you have to do this
for_each_possible_cpu() stuff:
1. when doing a r/w->r/o transition, which is rare, and
   certainly not a fast path
2. Where the per-cpu writer count underflows.  This requires
   a _minimum_ of 1<<16 file opens (configurable) each of which
   is closed on a different cpu than it was opened on.  Even
   if you were trying, I'm not sure you'd notice the overhead.

-- Dave

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