linux-kernel - Re: fsnotify_mark

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Message-ID: <CAJfpegvfuXHwmAx4pg+bOF-Dw5yJsa-7V5ACDeqBXG=WFtLE+g@mail.gmail.com>
Date:   Fri, 2 Dec 2016 09:26:51 +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 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.

Thanks,
Miklos