Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount

[Ian Kent <ikent@redhat.com>]

On 28/6/24 19:13, Jan Kara wrote:
> On Fri 28-06-24 10:58:54, Ian Kent wrote:
>> On 27/6/24 19:54, Jan Kara wrote:
>>> On Thu 27-06-24 09:11:14, Ian Kent wrote:
>>>> On 27/6/24 04:47, Matthew Wilcox wrote:
>>>>> On Wed, Jun 26, 2024 at 04:07:49PM -0400, Lucas Karpinski wrote:
>>>>>> +++ b/fs/namespace.c
>>>>>> @@ -78,6 +78,7 @@ static struct kmem_cache *mnt_cache __ro_after_init;
>>>>>>     static DECLARE_RWSEM(namespace_sem);
>>>>>>     static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
>>>>>>     static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
>>>>>> +static bool lazy_unlock = false; /* protected by namespace_sem */
>>>>> That's a pretty ugly way of doing it.  How about this?
>>>> Ha!
>>>>
>>>> That was my original thought but I also didn't much like changing all the
>>>> callers.
>>>>
>>>> I don't really like the proliferation of these small helper functions either
>>>> but if everyone
>>>>
>>>> is happy to do this I think it's a great idea.
>>> So I know you've suggested removing synchronize_rcu_expedited() call in
>>> your comment to v2. But I wonder why is it safe? I *thought*
>>> synchronize_rcu_expedited() is there to synchronize the dropping of the
>>> last mnt reference (and maybe something else) - see the comment at the
>>> beginning of mntput_no_expire() - and this change would break that?
>> Interesting, because of the definition of lazy umount I didn't look closely
>> enough at that.
>>
>> But I wonder, how exactly would that race occur, is holding the rcu read
>> lock sufficient since the rcu'd mount free won't be done until it's
>> released (at least I think that's how rcu works).
> I'm concerned about a race like:
>
> [path lookup]				[umount -l]
> ...
> path_put()
>    mntput(mnt)
>      mntput_no_expire(m)
>        rcu_read_lock();
>        if (likely(READ_ONCE(mnt->mnt_ns))) {
> 					do_umount()
> 					  umount_tree()
> 					    ...
> 					    mnt->mnt_ns = NULL;
> 					    ...
> 					  namespace_unlock()
> 					    mntput(&m->mnt)
> 					      mntput_no_expire(mnt)
> 				              smp_mb();
> 					      mnt_add_count(mnt, -1);
> 					      count = mnt_get_count(mnt);
> 					      if (count != 0) {
> 						...
> 						return;
>          mnt_add_count(mnt, -1);
>          rcu_read_unlock();
>          return;
> -> KABOOM, mnt->mnt_count dropped to 0 but nobody cleaned up the mount!
>        }
>
> And this scenario is exactly prevented by synchronize_rcu() in
> namespace_unlock().

I just wanted to say that I don't have a reply to this yet, having been 
distracted

looking at the concern that Christian raised, in fact this looks like it 
will be hard

to grok ...


Ian