* 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?
Thanks,
Ingo
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