From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) Subject: Re: [PATCH 0/1] ns: introduce proc_get_ns_by_fd() Date: Tue, 29 Sep 2015 14:05:42 -0500 Message-ID: <87si5xxczd.fsf@x220.int.ebiederm.org> References: <20150925135246.27620.97496.stgit@buzz> <20150925175654.GA12504@redhat.com> <871tdi8pqj.fsf@x220.int.ebiederm.org> <20150929164315.GA16734@redhat.com> <874mid16bk.fsf@x220.int.ebiederm.org> <20150929183833.GA21875@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150929183833.GA21875-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> (Oleg Nesterov's message of "Tue, 29 Sep 2015 20:38:33 +0200") List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Oleg Nesterov Cc: Roman Gushchin , Konstantin Khlebnikov , linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Serge Hallyn , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Chen Fan , Andrew Morton , Linus Torvalds List-Id: linux-api@vger.kernel.org Oleg Nesterov writes: > On 09/29, Eric W. Biederman wrote: >> >> Oleg Nesterov 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() ? Let me see. The call chain is a little interesting so let's trace it. thread1 thread2 net = net_ns_by_fd_rcu(fd); fput(file) get_net(net); ____fput __fput dput dentry_kill __dentry_kill dentry_iput iput iput_final evict ->evict_inode (nsfs_evict) netns_put put_net __put_net queue_work(net_cleanup_work) ... cleanup_net synchronize_rcu. Given that someone in another thread can put the file descriptor and I don't see anything explicitly waiting until after the rcu critical section ends to put the network namespace I guess we do need maybe_get_net. In practice I think the delay logic in fput will act as an rcu barrier, but that is not guaranteed so we had better not count on it. Sigh. Anyway this is a long ways from a syscall to translate pids. Eric