linux-kernel - Re: [PATCH 0/1] ns: introduce proc_get_ns_by

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Message-ID: <20150929183833.GA21875@redhat.com>
Date:	Tue, 29 Sep 2015 20:38:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Roman Gushchin <klamm@...dex-team.ru>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Chen Fan <chen.fan.fnst@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stéphane Graber <stgraber@...ntu.com>
Subject: Re: [PATCH 0/1] ns: introduce proc_get_ns_by_fd()

On 09/29, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > OK, I won't insist, this too looks better to me than proc_ns_fdget(&fd_ref).
> >
> > And in any case fcheck_files() makes more sense than fdget(), somehow I did
> > not think about this when I sent 1/1.
> >
> > Hmm. and after the quick look at cleanup_net() I can't understand whether
> > get_net_ns_by_fd() can use ns_by_fd_rcu() + maybe_get_net(to_net_ns()) or
> > not... Can it?
>
> Some of those places need a reference that allows them to sleep, and the
> code is shared with the legacy pid case so with an addition of get_net
> we can use ns_by_fd_rcu().   There are cases like setns that could
> use ns_by_fd_rcu() with code reording.
>
> We can implement get_net_ns_by_fd as:
> struct net *get_net_ns_by_fd(int fd)
> {
>         struct net *net;
>
> 	rcu_read_lock();
> 	net = net_ns_by_fd_rcu(fd);
>         if (!IS_ERR(net))
>         	get_net(net);
>         rcu_read_unlock();
>
> 	return net;
> }
>
> Which means we can achieve code sharing with the pure rcu version
> as a base.

Yes, this is what I meant... but don't we need maybe_get_net() ?

Oleg.

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