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

[Andrew Morton]

On Mon, 24 Sep 2007 12:28:11 -0700
Dave Hansen <haveblue@xxxxxxxxxx> wrote:

> 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.

And at include/linux/percpu_counter.h, please.

> 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.
> 

Sounds like what you're doing is more akin to the local_t-based module
refcounting.  `grep local_ kernel/module.c'.

That code should be converted from NR_CPUS to for_each_possible_cpu()..
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