linux-kernel - Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT

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Date: Fri, 28 Jun 2024 10:58:54 +0800
From: Ian Kent <ikent@...hat.com>
To: Jan Kara <jack@...e.cz>
Cc: Matthew Wilcox <willy@...radead.org>,
 Lucas Karpinski <lkarpins@...hat.com>, viro@...iv.linux.org.uk,
 brauner@...nel.org, raven@...maw.net, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, Alexander Larsson <alexl@...hat.com>,
 Eric Chanudet <echanude@...hat.com>
Subject: Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount


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).


In this case, when lazy is true, the mount will have been detached in 
umount_tree()

and mnt->mnt_ns set to NULL under the namespace sem write lock. So that 
condition

in mntput_no_expre() won't be true and the mount will no longer be found 
by the VFS.

I guess the question then becomes will any outstanding lockless path 
walks race with

this with only the rcu read lock to protect it, Christian?


Ian