On Wed, Jun 11, 2014 at 04:12:15PM -0700, Eric W. Biederman wrote:
> "Paul E. McKenney" <paul...@linux.vnet.ibm.com> writes:
>
> > On Wed, Jun 11, 2014 at 01:46:08PM -0700, Eric W. Biederman wrote:
> >> On the chance it is dropping the old nsproxy which calls syncrhonize_rcu
> >> in switch_task_namespaces that is causing you problems I have attached
> >> a patch that changes from rcu_read_lock to task_lock for code that
> >> calls task_nsproxy from a different task. The code should be safe
> >> and it should be an unquestions performance improvement but I have only
> >> compile tested it.
> >>
> >> If you can try the patch it will tell is if the problem is the rcu
> >> access in switch_task_namespaces (the only one I am aware of network
> >> namespace creation) or if the problem rcu case is somewhere else.
> >>
> >> If nothing else knowing which rcu accesses are causing the slow down
> >> seem important at the end of the day.
> >>
> >> Eric
> >>
> >
> > If this is the culprit, another approach would be to use workqueues from
> > RCU callbacks. The following (untested, probably does not even build)
> > patch illustrates one such approach.
>
> For reference the only reason we are using rcu_lock today for nsproxy is
> an old lock ordering problem that does not exist anymore.
>
> I can say that in some workloads setns is a bit heavy today because of
> the synchronize_rcu and setns is more important that I had previously
> thought because pthreads break the classic unix ability to do things in
> your process after fork() (sigh).
>
> Today daemonize is gone, and notify the parent process with a signal
> relies on task_active_pid_ns which does not use nsproxy. So the old
> lock ordering problem/race is gone.
>
> The description of what was happening when the code switched from
> task_lock to rcu_read_lock to protect nsproxy.
OK, never mind, then! ;-)
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.
>
> [c...@fr.ibm.com: fix unshare()]
> [ebied...@xmission.com: Update get_net_ns_by_pid]
> Signed-off-by: Pavel Emelyanov <xe...@openvz.org>
> Signed-off-by: Eric W. Biederman <ebied...@xmission.com>
> Cc: Oleg Nesterov <o...@tv-sign.ru>
> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: Serge Hallyn <se...@us.ibm.com>
> Signed-off-by: Cedric Le Goater <c...@fr.ibm.com>
> Signed-off-by: Andrew Morton <a...@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
>
> Eric
>
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