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Date: Wed, 5 Aug 2015 13:38:54 +0200
From: Ingo Molnar <mingo@kernel.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, David Howells <dhowells@redhat.com>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness
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* Kirill A. Shutemov <kirill@shutemov.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@chromium.org> wrote:
> > 
> > > From: Ricky Zhou <rickyz@chromium.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