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@xxxxxxxxxx> 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);
> + }
> }
>
> /*
>
