On Mon, Feb 17, 2020 at 10:23:18AM +0100, Giuseppe Scrivano wrote:
> Hi Paul,
>
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:
>
> > Nice speedup!
> >
> > However, I am not convinced that the code shown below is safe.
> >
> > I believe that you need either a synchronize_rcu() in your free_ipc()
> > function or that you need to pass free_ipc() to queue_rcu_work() instead
> > of directly schedule_work(). As things are, I would expect you to see
> > free_ipc_ns() being invoke too soon on heavily loaded CONFIG_PREEMPT=y
> > kernels. Which can be quite a pain to debug!
> >
> > Or am I missing something?
>
> thanks for the review!
>
> Would being called too soon be a problem? The current version calls
> free_ipc_ns() as part of the put_ipc_ns cleanup.
>
> free_ipc() calls immediately synchronize_rcu() through free_ipc_ns():
>
> free_ipc_ns() -> mq_put_mnt() -> kern_unmount() -> synchronize_rcu()
Ah, I did not look deeply enough.
> Do you think we should make it more explicit and add a synchronize_rcu()
> as part of the free_ipc_ns() function itself?
Maybe just a comment on the free_ipc_ns() saying that it waits for
a grace period.
Thanx, Paul
> Regards,
> Giuseppe
>
>
> >
> > Thanx, Paul
> >
> >> Signed-off-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
> >> ---
> >> include/linux/ipc_namespace.h | 2 ++
> >> ipc/namespace.c | 17 +++++++++++++++--
> >> 2 files changed, 17 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> >> index c309f43bde45..a06a78c67f19 100644
> >> --- a/include/linux/ipc_namespace.h
> >> +++ b/include/linux/ipc_namespace.h
> >> @@ -68,6 +68,8 @@ struct ipc_namespace {
> >> struct user_namespace *user_ns;
> >> struct ucounts *ucounts;
> >>
> >> + struct llist_node mnt_llist;
> >> +
> >> struct ns_common ns;
> >> } __randomize_layout;
> >>
> >> diff --git a/ipc/namespace.c b/ipc/namespace.c
> >> index b3ca1476ca51..37d27e1b807a 100644
> >> --- a/ipc/namespace.c
> >> +++ b/ipc/namespace.c
> >> @@ -117,6 +117,7 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
> >>
> >> static void free_ipc_ns(struct ipc_namespace *ns)
> >> {
> >> + mq_put_mnt(ns);
> >> sem_exit_ns(ns);
> >> msg_exit_ns(ns);
> >> shm_exit_ns(ns);
> >> @@ -127,6 +128,17 @@ static void free_ipc_ns(struct ipc_namespace *ns)
> >> kfree(ns);
> >> }
> >>
> >> +static LLIST_HEAD(free_ipc_list);
> >> +static void free_ipc(struct work_struct *unused)
> >> +{
> >> + struct llist_node *node = llist_del_all(&free_ipc_list);
> >> + struct ipc_namespace *n, *t;
> >> +
> >> + llist_for_each_entry_safe(n, t, node, mnt_llist)
> >> + free_ipc_ns(n);
> >> +}
> >> +static DECLARE_WORK(free_ipc_work, free_ipc);
> >> +
> >> /*
> >> * put_ipc_ns - drop a reference to an ipc namespace.
> >> * @ns: the namespace to put
> >> @@ -148,8 +160,9 @@ void put_ipc_ns(struct ipc_namespace *ns)
> >> if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
> >> mq_clear_sbinfo(ns);
> >> spin_unlock(&mq_lock);
> >> - mq_put_mnt(ns);
> >> - free_ipc_ns(ns);
> >> +
> >> + if (llist_add(&ns->mnt_llist, &free_ipc_list))
> >> + schedule_work(&free_ipc_work);
> >> }
> >> }
> >>
> >> --
> >> 2.24.1
> >>
>
