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

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Message-ID: <20150805113854.GA9110@gmail.com>
Date:	Wed, 5 Aug 2015 13:38:54 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
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


* 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?

Thanks,

	Ingo
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