linux-kernel - Re: [RFC PATCH 0/7] Inotify support in FUSE and virtiofs

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Message-ID: <CAOQ4uxhpCKK2MYxSmRJYYMEWaHKy5ezyKgxaM+YAKtpjsZkD-g@mail.gmail.com>
Date:   Tue, 26 Oct 2021 20:59:44 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Ioannis Angelakopoulos <iangelak@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        virtio-fs-list <virtio-fs@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Steve French <sfrench@...ba.org>
Subject: Re: [RFC PATCH 0/7] Inotify support in FUSE and virtiofs

On Tue, Oct 26, 2021 at 7:18 PM Vivek Goyal <vgoyal@...hat.com> wrote:
>
> On Tue, Oct 26, 2021 at 06:23:50PM +0300, Amir Goldstein wrote:
>
> [..]
> > > 3) The lifetime of the local watch in the guest kernel is very
> > > important. Specifically, there is a possibility that the guest does not
> > > receive remote events on time, if it removes its local watch on the
> > > target or deletes the inode (and thus the guest kernel removes the watch).
> > > In these cases the guest kernel removes the local watch before the
> > > remote events arrive from the host (virtiofsd) and as such the guest
> > > kernel drops all the remote events for the target inode (since the
> > > corresponding local watch does not exist anymore).
>
> So this is one of the issues which has been haunting us in virtiofs. If
> a file is removed, for local events, event is generated first and
> then watch is removed. But in case of remote filesystems, it is racy.
> It is possible that by the time event arrives, watch is already gone
> and application never sees the delete event.
>
> Not sure how to address this issue.

Can you take me through the scenario step by step.
I am not sure I understand the exact sequence of the race.
If it is local file removal that causes watch to be removed,
then don't drop local events and you are good to go.
Is it something else?

Thanks,
Amir.