On Wed, Aug 05, 2015 at 01:38:54PM +0200, Ingo Molnar wrote:
>
> * Kirill A. Shutemov <kir...@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 <keesc...@chromium.org>
> > > wrote:
> > >
> > > > From: Ricky Zhou <ric...@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
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