Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

Ian Kent
2025-04-18T01:16:12Z

On 18/4/25 00:28, Christian Brauner wrote:
> On Thu, Apr 17, 2025 at 05:31:26PM +0200, Sebastian Andrzej Siewior wrote:
>> On 2025-04-17 17:28:20 [+0200], Christian Brauner wrote:
>>>> So if there's some userspace process with a broken NFS server and it
>>>> does umount(MNT_DETACH) it will end up hanging every other
>>>> umount(MNT_DETACH) on the system because the dealyed_mntput_work
>>>> workqueue (to my understanding) cannot make progress.
>>> Ok, "to my understanding" has been updated after going back and reading
>>> the delayed work code. Luckily it's not as bad as I thought it is
>>> because it's queued on system_wq which is multi-threaded so it's at
>>> least not causing everyone with MNT_DETACH to get stuck. I'm still
>>> skeptical how safe this all is.
>> I would (again) throw system_unbound_wq into the game because the former
>> will remain on the CPU on which has been enqueued (if speaking about
>> multi threading).
> Yes, good point.
>
> However, what about using polled grace periods?
>
> A first simple-minded thing to do would be to record the grace period
> after umount_tree() has finished and the check it in namespace_unlock():
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index d9ca80dcc544..1e7ebcdd1ebc 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -77,6 +77,7 @@ static struct hlist_head *mount_hashtable __ro_after_init;
> static struct hlist_head *mountpoint_hashtable __ro_after_init;
> static struct kmem_cache *mnt_cache __ro_after_init;
> static DECLARE_RWSEM(namespace_sem);
> +static unsigned long rcu_unmount_seq; /* protected by namespace_sem */
> static HLIST_HEAD(unmounted); /* protected by namespace_sem */
> static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
> static DEFINE_SEQLOCK(mnt_ns_tree_lock);
> @@ -1794,6 +1795,7 @@ static void namespace_unlock(void)
> struct hlist_head head;
> struct hlist_node *p;
> struct mount *m;
> + unsigned long unmount_seq = rcu_unmount_seq;
> LIST_HEAD(list);
>
> hlist_move_list(&unmounted, &head);
> @@ -1817,7 +1819,7 @@ static void namespace_unlock(void)
> if (likely(hlist_empty(&head)))
> return;
>
> - synchronize_rcu_expedited();
> + cond_synchronize_rcu_expedited(unmount_seq);
>
> hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
> hlist_del(&m->mnt_umount);
> @@ -1939,6 +1941,8 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
> */
> mnt_notify_add(p);
> }
> +
> + rcu_unmount_seq = get_state_synchronize_rcu();
> }
>
> static void shrink_submounts(struct mount *mnt);
>
>
> I'm not sure how much that would buy us. If it doesn't then it should be
> possible to play with the following possibly strange idea:
>
> diff --git a/fs/mount.h b/fs/mount.h
> index 7aecf2a60472..51b86300dc50 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -61,6 +61,7 @@ struct mount {
> struct rb_node mnt_node; /* node in the ns->mounts rbtree */
> struct rcu_head mnt_rcu;
> struct llist_node mnt_llist;
> + unsigned long mnt_rcu_unmount_seq;
> };
> #ifdef CONFIG_SMP
> struct mnt_pcp __percpu *mnt_pcp;
> diff --git a/fs/namespace.c b/fs/namespace.c
> index d9ca80dcc544..aae9df75beed 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1794,6 +1794,7 @@ static void namespace_unlock(void)
> struct hlist_head head;
> struct hlist_node *p;
> struct mount *m;
> + bool needs_synchronize_rcu = false;
> LIST_HEAD(list);
>
> hlist_move_list(&unmounted, &head);
> @@ -1817,7 +1818,16 @@ static void namespace_unlock(void)
> if (likely(hlist_empty(&head)))
> return;
>
> - synchronize_rcu_expedited();
> + hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
> + if (!poll_state_synchronize_rcu(m->mnt_rcu_unmount_seq))
> + continue;
> +
> + needs_synchronize_rcu = true;
> + break;
> + }
> +
> + if (needs_synchronize_rcu)
> + synchronize_rcu_expedited();
>
> hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
> hlist_del(&m->mnt_umount);
> @@ -1923,8 +1933,10 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
> }
> }
> change_mnt_propagation(p, MS_PRIVATE);
> - if (disconnect)
> + if (disconnect) {
> + p->mnt_rcu_unmount_seq = get_state_synchronize_rcu();
> hlist_add_head(&p->mnt_umount, &unmounted);
> + }
>
> /*
> * At this point p->mnt_ns is NULL, notification will be queued
>
> This would allow to elide synchronize rcu calls if they elapsed in the
> meantime since we moved that mount to the unmounted list.

This last patch is a much better way to do this IMHO.

The approach is so much more like many other places we have "rcu check
before use" type code.

Ian