Re: [PATCH] ipc: use a work queue to free_ipc

[Paul E. McKenney]

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
> >> 
>