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

Ian Kent
2025-04-18T01:22:22Z

On 18/4/25 09:13, Ian Kent wrote:
>
> 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.

If there are several thousand mounts in the discard list having two
loops could end up a bit slow.

I wonder if we could combine the two loops into one ... I'll think about
that.

>
> Ian
>
>