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Date: Wed, 5 Aug 2015 14:53:19 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Ingo Molnar <mingo@kernel.org>
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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On Wed, Aug 05, 2015 at 01:38:54PM +0200, Ingo Molnar wrote:
> 
> * 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?

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