On Tue, Oct 22, 2013 at 12:44:20PM -0700, Eric W. Biederman wrote:
>
> To be succint.
>
> Mutation of nsproxy in place was a distraction.
>
> What is crucial to the current operation of the code is
>
> synchronize_rcu();
> put_pid_ns();
> put_net_ns();
> ...
>
> To remove the syncrhonize_rcu we would have to either user call_rcu or
> make certain all of the namespaces have some kind of rcu liveness
> guarantee (which many of them do) and use something like maybe_get_net.
You beat me to the call_rcu() suggestion. If the callback needs to
do something that might sleep, the usual trick is a workqueue scheduled
from the RCU callback.
> If you are going to pursue this the maybe_get_net direction is my
> preference as that is what we would need if we did not have nsproxy
> and so will be simpler overall.
>
> Hmm. On the side of simple it may be appropriate to revisit the patch
> that started using rcu protection for nsproxy. I doesn't look like
> the original reasons for nsproxy being rcu protected exist any more,
> so reverting to task_lock protect may be enough..
>
> And it would result in faster/simpler code that only slows down when we
> perform a remote access, which should be far from common.
That can be a good option, also. ;-)
Thanx, Paul
> commit cf7b708c8d1d7a27736771bcf4c457b332b0f818
> Author: Pavel Emelyanov <xe...@openvz.org>
> Date: Thu Oct 18 23:39:54 2007 -0700
>
> Make access to task's nsproxy lighter
>
> When someone wants to deal with some other taks's namespaces it has to
> lock
> the task and then to get the desired namespace if the one exists. This is
> slow on read-only paths and may be impossible in some cases.
>
> E.g. Oleg recently noticed a race between unshare() and the (sent for
> review in cgroups) pid namespaces - when the task notifies the parent it
> has to know the parent's namespace, but taking the task_lock() is
> impossible there - the code is under write locked tasklist lock.
>
> On the other hand switching the namespace on task (daemonize) and
> releasing
> the namespace (after the last task exit) is rather rare operation and we
> can sacrifice its speed to solve the issues above.
>
> The access to other task namespaces is proposed to be performed
> like this:
>
> rcu_read_lock();
> nsproxy = task_nsproxy(tsk);
> if (nsproxy != NULL) {
> / *
> * work with the namespaces here
> * e.g. get the reference on one of them
> * /
> } / *
> * NULL task_nsproxy() means that this task is
> * almost dead (zombie)
> * /
> rcu_read_unlock();
>
> This patch has passed the review by Eric and Oleg :) and,
> of course, tested.
>
>
> Eric
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
