[PATCH v2] ns: do not block exit_task_namespaces() for a long time

[Al Viro]

On Mon, Jul 16, 2012 at 08:16:34PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jul 16, 2012 at 05:53:01PM +0100, Al Viro wrote:
> > On Mon, Jul 16, 2012 at 06:09:24PM +0300, Kirill A. Shutemov wrote:
> > > From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> > > 
> > > On exiting of the last task in a namespace we need to trigger freeing of
> > > the namespace. Currently, we call synchronize_rcu() and free_nsproxy()
> > > directly on do_exit() path.
> > > 
> > > On my machine synchronize_rcu() blocks for about 0.01 seconds. For
> > > comparing: normal exit_group() syscall takes less than 0.0003 seconds.
> > > 
> > > Let's offload synchronize_rcu() and free_nsproxy() to a workqueue.
> > > 
> > > I also move synchronize_rcu() inside free_nsproxy(). It fixes racy
> > > put_nsproxy() which calls free_nsproxy() without synchronize_rcu().
> > > I guess it was missed during switch to RCU (see cf7b708).
> > 
> > NAK.  Making final umounts of anything in that namespace asynchronous,
> > even though nothing is holding the stuff on them busy is simply
> > wrong.  Note that they can take a _long_ time, so we are talking about
> > minutes worth of delay in the worst case.  It's user-visible and
> > it's a serious potential for trouble.
> 
> Good point.
> 
> Now in worst case we have a process which hang for a few minutes in
> exit_group() syscall in D state, right? Why is that any better?
> Does it provide better user experience or better accounting or what?

"Session that was using that USB stick has still not finished exiting;
might be still busy writing stuff there, so better not pull it out yet".