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

From: Ricky Zhou <ricky@rzhou.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Ingo Molnar <mingo@kernel.org>,
	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
Date: Wed, 5 Aug 2015 06:13:22 -0700	[thread overview]
Message-ID: <CAFoVXjjoV+fhEA1iwWasVa0UgHZjXq9uzGS0LqQmFwMzCwkq_A@mail.gmail.com> (raw)
In-Reply-To: <20150805115319.GB25784@node.dhcp.inet.fi>

On Wed, Aug 5, 2015 at 4:53 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>> '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.
To be fair, the entire reason for this patch is that the slow path
(mm_users > 1) can happen even when the process is single-threaded. I
was concerned about how expensive current_is_single_threaded looked as
well, but didn't see any lightweight alternatives short of adding a
field to mm_struct. Do folks think it's worth going down that route
instead?

Thanks,
Ricky