Re: [RFC][PATCH 2/2] fsnotify: handle permission events without holding fsnotify_mark_srcu[0]

From: Jan Kara
Date: Wed Nov 16 2016 - 04:35:47 EST

On Mon 14-11-16 17:09:47, Amir Goldstein wrote:
> On Mon, Nov 14, 2016 at 3:20 PM, Jan Kara <jack@xxxxxxx> wrote:
> > On Mon 14-11-16 13:48:27, Amir Goldstein wrote:
> >> Handling fanotify events does not entail dereferencing fsnotify_mark
> >> beyond the point of fanotify_should_send_event().
> >>
> >> For the case of permission events, which may block indefinitely,
> >> return -EAGAIN and then fsnotify() will call handle_event() again
> >> without a reference to the mark.
> >>
> >> Without a reference to the mark, there is no need to call
> >> handle_event() under fsnotify_mark_srcu[0] read side lock,
> >> so we drop fsnotify_mark_srcu[0] while handling the event
> >> and grab it back before continuing to the next mark.
> >>
> >> After this change, a blocking permission event will no longer
> >> block closing of any file descriptors of 0 priority groups,
> >> i.e: inotify and fanotify groups of class FAN_CLASS_NOTIF.
> >>
> >> Reported-by: Miklos Szeredi <miklos@xxxxxxxxxx>
> >> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> >
> > Well, this has a similar problem as my attempt to fix the issue. The
> > current mark can get removed from the mark list while waiting for userspace
> > response. ->next pointer is still valid at that moment but ->next->pprev
> > already points to mark preceding us (that's how rcu lists work). When
> > ->next mark then gets removed from the list and destroyed (it may be
> > protected by the second srcu), our ->next pointer points to freed memory.
> Oh! I missed the fact that the SRCU does not protect mark->obj_list.
> Can resurrecting mark->free_list buy us anything?
> Perhaps keep the marks on obj_list without FLAG_ATTACHED
> and then remove marks from both free_list and obj_list post srcu_synchronize()?
> I think that removing mark->obj_list was optimization of good faith
> and not because it really hurts the system's memory usage that much?

You have to be really careful here. Minimally you'd need then another
srcu_synchronize() call after removing mark from the list and before
freeing the mark so that you are sure no process iterating the list can
have a reference to a mark being freed but even then it needs careful
checking whether it would work. The joy of lockless algorithms...

> > I have some ideas how to fix up issues with my refcounting approach -
> > refcount would pin marks not only in memory but also in lists but I have
> > yet to see whether that works out sensibly (it would mean that dropping
> > mark reference would then need to take group->mark_mutex and that may cause
> > lock ordering issues).
> It sounds like a chicken and egg problem, but I suppose you don't mean
> the actual mark refcount, but a secondary "pinned refcount"?

No, I mean the original refcount. I have patches that already postpone part
of the mark cleanup to happen only once the refcount drops to 0.

> Anyway, if you have something ready, my test setup is already in place..

Thanks for an offer. I don't have anything yet, hopefully later today or

Jan Kara <jack@xxxxxxxx>