linux-kernel - Re: fsnotify_mark

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Message-ID: <20161202104804.GC26086@quack2.suse.cz>
Date:   Fri, 2 Dec 2016 11:48:04 +0100
From:   Jan Kara <jack@...e.cz>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Jan Kara <jack@...e.cz>, 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 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.).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR