linux-kernel - Re: [PATCH v2 2/3] fsnotify: Protect i_fsnotify

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Date:   Tue, 25 Oct 2022 11:02:36 -0700
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] fsnotify: Protect i_fsnotify_mask and child
 flags with inode rwsem

Amir Goldstein <amir73il@...il.com> writes:

> On Fri, Oct 21, 2022 at 11:22 AM Amir Goldstein <amir73il@...il.com> wrote:
> ...
>> > +/*
>> > + * Objects may need some additional actions to be taken when the last reference
>> > + * is dropped. Define flags to indicate which actions are necessary.
>> > + */
>> > +#define FSNOTIFY_OBJ_FLAG_NEED_IPUT            0x01
>> > +#define FSNOTIFY_OBJ_FLAG_UPDATE_CHILDREN      0x02
>>
>> with changed_flags argument, you do not need these, you can use
>> the existing CONN_FLAGS.
>>
>> It is a bit ugly that the direction of the change is not expressed
>> in changed_flags, but for the current code, it is not needed, because
>> update_children does care about the direction of the change and
>> the direction of change to HAS_IREF is expressed by the inode
>> object return value.
>>
>
> Oh that is a lie...
>
> return value can be non NULL because of an added mark
> that wants iref and also wants to watch children, but the
> only practical consequence of this is that you can only
> do the WARN_ON for the else case of update_children
> in fsnotify_recalc_mask().
>
> I still think it is a win for code simplicity as I detailed
> in my comments.
>
>> Maybe try it out in v3 to see how it works.
>>
>> Unless Jan has an idea that will be easier to read and maintain...
>>
>
> Maybe fsnotify_update_inode_conn_flags() should return "update_flags"
> and not "changed_flags", because actually the WATCHING_CHILDREN
> flag is not changed by the helper itself.

Yeah, this is the way I'd like to go. The approach of "orig_flags ^
new_flags" doesn't work since we're not changing the WATCHING_CHILDREN
flag.

At the end of the day, I do believe that it's equivalent to what I had
originally, except that we'd use FSNOTIFY_CONN_FLAG_* rather than my new
FSNOTIFY_OBJ_FLAG_*, which works for me, the new constants are a bit of
clutter.

> Then, HAS_IREF is not returned when helper did get_iref() and changed
> HAS_IREF itself and then the comment that says:
>      /* Unpin inode after detach of last mark that wanted iref */
> will be even clearer:
>
>         if (want_iref) {
>                 /* Pin inode if any mark wants inode refcount held */
>                 fsnotify_get_inode_ref(fsnotify_conn_inode(conn));
>                 conn->flags |= FSNOTIFY_CONN_FLAG_HAS_IREF;
>         } else {
>                 /* Unpin inode after detach of last mark that wanted iref */
>                 ret = inode;
>                 update_flags |= FSNOTIFY_CONN_FLAG_HAS_IREF;

Is it possible that once the spinlock is dropped, another
fsnotify_recalc_mask() finds that FSNOTIFY_CONN_FLAG_HAS_IREF is still
set, and so it also sets FSNOTIFY_CONN_FLAG_HAS_IREF, causing two
threads to both do an iput?

It may not be possible due to the current use of the functions, but I
guess it would be safer to clear the connector flag here under the
spinlock, and set the *update_flags accordingly so that only one thread
performs the iput().


Stephen

>         }
>
> Thanks,
> Amir.