Messages in this thread | | | From | (Eric W. Biederman) | Date | Tue, 29 Sep 2015 14:05:42 -0500 | Subject | Re: [PATCH 0/1] ns: introduce proc_get_ns_by_fd() |
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Oleg Nesterov <oleg@redhat.com> writes:
> On 09/29, Eric W. Biederman wrote: >> >> Oleg Nesterov <oleg@redhat.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() ?
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
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