linux-kernel - Re: [PATCH] user_ns: use correct check for single-threadedness

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Date:	Wed, 5 Aug 2015 14:53:19 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Ricky Zhou <rickyz@...omium.org>,
	Julien Tinnes <jln@...gle.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness

On Wed, Aug 05, 2015 at 01:38:54PM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill@...temov.name> wrote:
> 
> > On Tue, Jul 28, 2015 at 02:35:04PM -0700, Andrew Morton wrote:
> > > On Tue, 28 Jul 2015 10:15:00 -0700 Kees Cook <keescook@...omium.org> wrote:
> > > 
> > > > From: Ricky Zhou <rickyz@...omium.org>
> > > > 
> > > > Checking mm_users > 1 does not mean a process is multithreaded. For
> > > > example, reading /proc/PID/maps temporarily increments mm_users, allowing
> > > > other processes to (accidentally) interfere with unshare() calls.
> > > > 
> > > > This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
> > > > returning EINVAL if another processes happened to be simultaneously
> > > > reading the maps file.
> > > 
> > > Yikes.  current_is_single_threaded() is expensive.  Are we sure this
> > > isn't going to kill someone's workload?
> > 
> > It's expensive only if mm_users > 1. We will go to for_each_process() only
> > if somebody outside of the process grabs mm_users references (like reading
> > /proc/PID/maps). Or if it called it from multithreaded application.
> 
> It's considerably expensive:
> 
>         rcu_read_lock();
>         for_each_process(p) {
>                 do {
> 	...
>                 } while_each_thread(p, t);
>         }
> 
> 
> 'only' if it's multi-threaded, i.e. when some workload cares so much about 
> performance that it uses multiple threads?
> 
> Can you see the contradiction there?

I can. man 2 unshare:

	CLONE_NEWUSER requires that the calling process  is  not  threaded;

The workload cares so much about performance that it ignores API
requirements. Some slow down looks like a fair price to me.

-- 
 Kirill A. Shutemov
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