linux-kernel - Re: [PATCH v2] ns: do not block exit_task

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20120716171634.GA21620@shutemov.name>
Date:	Mon, 16 Jul 2012 20:16:34 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Dmitry V. Levin" <ldv@...linux.org>,
	Doug Ledford <dledford@...hat.com>,
	linux-kernel@...r.kernel.org, containers@...ts.linux-foundation.org
Subject: Re: [PATCH v2] ns: do not block exit_task_namespaces() for a long
 time

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@...ux.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?

-- 
 Kirill A. Shutemov
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/