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

[Ian Kent <ikent@redhat.com>]

On 3/7/24 17:22, Christian Brauner wrote:
> On Mon, Jul 01, 2024 at 02:10:31PM GMT, Christian Brauner wrote:
>> On Mon, Jul 01, 2024 at 10:41:40AM GMT, Alexander Larsson wrote:
>>> On Mon, Jul 1, 2024 at 7:50 AM Christian Brauner <brauner@kernel.org> wrote:
>>>>> I always thought the rcu delay was to ensure concurrent path walks "see" the
>>>>>
>>>>> umount not to ensure correct operation of the following mntput()(s).
>>>>>
>>>>>
>>>>> Isn't the sequence of operations roughly, resolve path, lock, deatch,
>>>>> release
>>>>>
>>>>> lock, rcu wait, mntput() subordinate mounts, put path.
>>>> The crucial bit is really that synchronize_rcu_expedited() ensures that
>>>> the final mntput() won't happen until path walk leaves RCU mode.
>>>>
>>>> This allows caller's like legitimize_mnt() which are called with only
>>>> the RCU read-lock during lazy path walk to simple check for
>>>> MNT_SYNC_UMOUNT and see that the mnt is about to be killed. If they see
>>>> that this mount is MNT_SYNC_UMOUNT then they know that the mount won't
>>>> be freed until an RCU grace period is up and so they know that they can
>>>> simply put the reference count they took _without having to actually
>>>> call mntput()_.
>>>>
>>>> Because if they did have to call mntput() they might end up shutting the
>>>> filesystem down instead of umount() and that will cause said EBUSY
>>>> errors I mentioned in my earlier mails.
>>> But such behaviour could be kept even without an expedited RCU sync.
>>> Such as in my alternative patch for this:
>>> https://www.spinics.net/lists/linux-fsdevel/msg270117.html
>>>
>>> I.e. we would still guarantee the final mput is called, but not block
>>> the return of the unmount call.
>> That's fine but the patch as sent doesn't work is my point. It'll cause
>> exactly the issues described earlier, no? So I'm confused why this
>> version simply ended up removing synchronize_rcu_expedited() when
>> the proposed soluton seems to have been to use queue_rcu_work().
>>
>> But anyway, my concern with this is still that this changes the way
>> MNT_DETACH behaves when you shut down a non-busy filesystem with
>> MNT_DETACH as outlined in my other mail.
>>
>> If you find a workable version I'm not entirely opposed to try this but
>> I wouldn't be surprised if this causes user visible issues for anyone
>> that uses MNT_DETACH on a non-used filesystem.
> Correction: I misremembered that umount_tree() is called with
> UMOUNT_SYNC only in the case that umount() isn't called with MNT_DETACH.
> I mentioned this yesterday in the thread but just in case you missed it
> I want to spell it out in detail as well.

Thanks Christian, I did see that, yep.

There's also the seqlock in there to alert the legitimize that it needs

to restart using ref-walk.


>
> This is relevant because UMOUNT_SYNC will raise MNT_SYNC_UMOUNT on all
> mounts it unmounts. And that ends up being checked in legitimize_mnt()
> to ensure that legitimize_mnt() doesn't call mntput() during path lookup
> and risking EBUSY for a umount(..., 0) + mount() sequence for the same
> filesystem.
>
> But for umount(.., MNT_DETACH) UMOUNT_SYNC isn't passed and so
> MNT_SYNC_UMOUNT isn't raised on the mount and so legitimize_mnt() may
> end up doing the last mntput() and cleaning up the filesystem.
>
> In other words, a umount(..., MNT_DETACH) caller needs to be prepared to
> deal with EBUSY for a umount(..., MNT_DETACH) + mount() sequence.
>
> So I think we can certainly try this as long as we make it via
> queue_rcu_work() to handle the other mntput_no_expire() grace period
> dependency we discussed upthread.
>
> Thanks for making take a closer look.

I'm still not sure I fully understand the subtleties of how this works, I

think I'll need to do a deep dive into the rcu code and then revisit the

umount code. At least I won't be idle, ;(


Nevertheless I have to thank both you and Honza for your efforts and 
tolerance.


Ian

Ian