linux-kernel - Re: fsnotify_mark

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Message-ID: <CAJfpegvh2RQjoUNjZc4Y6UNhgvWWLG4YpeyC_BET24qfNY2yZg@mail.gmail.com>
Date:   Fri, 2 Dec 2016 12:02:33 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Jan Kara <jack@...e.cz>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Eric Paris <eparis@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: fsnotify_mark_srcu wtf?

On Fri, Dec 2, 2016 at 11:48 AM, Jan Kara <jack@...e.cz> wrote:
> On Fri 02-12-16 09:26:51, Miklos Szeredi wrote:
>> On Thu, Nov 10, 2016 at 8:46 PM, Jan Kara <jack@...e.cz> wrote:
>> > On Wed 09-11-16 20:26:16, Amir Goldstein wrote:
>> >> On Wed, Nov 9, 2016 at 1:10 PM, Jan Kara <jack@...e.cz> wrote:
>>
>> >> > And this does not work as well... Fanotify must notify groups by their
>> >> > priority so you cannot arbitrarily reorder ordering in which groups get
>> >> > notified. I'm currently pondering on using mark refcount to pin it when
>> >> > processing permission event but there are still some details to check.
>> >> >
>> >>
>> >> All right, mark refcount sound like the proper solution.
>> >
>> > Except it doesn't quite work. We can pin the current marks by a refcount
>> > but they can still be removed from the list so after we regain srcu lock,
>> > we are not sure their ->next pointers still point to still allocated marks
>> > :-| Sadly I realized this only after implementing all this.
>>
>> Hmm, how about this: when removing mark from inode, drop refcount.  If
>> refcount is zero can remove from list.  Otherwise mark the mark "dead"
>> and leave it on the list.
>>
>> And fsnotify can just skip dead marks.
>
> I had this idea as well and when trying to implement this, I've stumbled
> over some problems. I think the biggest problem was that destruction of a
> notification mark is relatively complex operation (doing iput() for
> example) and quite a few places dropping mark references are in a context
> where this can cause problems. Also I don't want to defer iput() to a
> workqueue as that will have unexpected consequences such as unlinked
> watched inode lingering in the system (possibly colliding with umount
> etc.).

Okay, but all we need from the deleted mark is the ->next pointer, no?
 So everything else related to destruction can be done.

Thanks,
Miklos