linux-kernel - Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.

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Date:	Fri, 31 Oct 2008 10:16:12 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	ncunningham@...a.org.au
CC:	miklos@...redi.hu, stern@...land.harvard.edu,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] Freezer: Don't count threads waiting for frozen
	filesystems.

On Fri, 31 Oct 2008, Nigel Cunningham wrote:
> Hi.
> 
> On Fri, 2008-10-31 at 09:49 +0100, Miklos Szeredi wrote:
> > On Fri, 31 Oct 2008, Nigel Cunningham wrote:
> > > I'm not sure that's true. You see, I'm thinking of this as not that
> > > different to the problem of unmounting filesystems. There, too, we need
> > > to unmount in a particular order, and let transactions on each
> > > filesystem stop cleanly before we can unmount them. Even if there are
> > > differences, perhaps looking at how we handle unmounting will help with
> > > handling freezing.
> > 
> > There's nothing magic about umount, it just uses a refcount on the fs.
> > 
> > But umount changes the namespace, that's the big difference.  For
> > example if a process is accessing path P which has a component inside
> > the mount, it _will_ get different results before and after the
> > umount.  This is not acceptable for freezing.
> > 
> > For freezing to work with such a refcounting scheme, we'd have to
> > count _future_ uses of the fs as well, not just current ones, which is
> > obviously impossible.
> 
> I must be missing something. If you're freezing future users of the
> filesystem before they can start anything new, doesn't that deal with
> this problem?

How do you determine which are the future users?

Miklos
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