Re: [PATCH RFC fs/namespace] Make kern_unmount() use synchronize_rcu_expedited()

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Feb 14, 2022 at 03:55:36PM -0500, Rik van Riel wrote:
> On Mon, 14 Feb 2022 11:44:40 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > On Mon, Feb 14, 2022 at 07:26:49PM +0000, Chris Mason wrote:
> 
> > Moving from synchronize_rcu() to synchronize_rcu_expedited() does buy
> > you at least an order of magnitude.  But yes, it should be possible to
> > get rid of all but one call per batch, which would be better.  Maybe
> > a bit more complicated, but probably not that much.
> 
> It doesn't look too bad, except for the include of ../fs/mount.h.
> 
> I'm hoping somebody has a better idea on how to deal with that.
> Do we need a kern_unmount() variant that doesn't do the RCU wait,
> or should it get a parameter, or something else?
> 
> Is there an ordering requirement between the synchronize_rcu call
> and zeroing out n->mq_mnt->mnt_ls?
> 
> What other changes do we need to make everything right?
> 
> The change below also fixes the issue that to-be-freed items that
> are queued up while the free_ipc work function runs do not result
> in the work item being enqueued again.
> 
> This patch is still totally untested because the 4 year old is
> at home today :)

I agree that this should decrease grace-period latency quite a bit
more than does my patch, so here is hoping that it does work.  ;-)

							Thanx, Paul

> diff --git a/ipc/namespace.c b/ipc/namespace.c
> index 7bd0766ddc3b..321cbda17cfb 100644
> --- a/ipc/namespace.c
> +++ b/ipc/namespace.c
> @@ -17,6 +17,7 @@
>  #include <linux/proc_ns.h>
>  #include <linux/sched/task.h>
>  
> +#include "../fs/mount.h"
>  #include "util.h"
>  
>  static struct ucounts *inc_ipc_namespaces(struct user_namespace *ns)
> @@ -117,10 +118,7 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
>  
>  static void free_ipc_ns(struct ipc_namespace *ns)
>  {
> -	/* mq_put_mnt() waits for a grace period as kern_unmount()
> -	 * uses synchronize_rcu().
> -	 */
> -	mq_put_mnt(ns);
> +	mntput(ns->mq_mnt);
>  	sem_exit_ns(ns);
>  	msg_exit_ns(ns);
>  	shm_exit_ns(ns);
> @@ -134,11 +132,19 @@ static void free_ipc_ns(struct ipc_namespace *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 llist_node *node;
>  	struct ipc_namespace *n, *t;
>  
> -	llist_for_each_entry_safe(n, t, node, mnt_llist)
> -		free_ipc_ns(n);
> +	while ((node = llist_del_all(&free_ipc_list))) {
> +		llist_for_each_entry(n, node, mnt_llist)
> +			real_mount(n->mq_mnt)->mnt_ns = NULL;
> +
> +		/* Wait for the last users to have gone away. */
> +		synchronize_rcu();
> +
> +		llist_for_each_entry_safe(n, t, node, mnt_llist)
> +			free_ipc_ns(n);
> +	}
>  }
>  
>  /*
>