Re: Things I wish I'd known about Inotify

[Jan Kara <jack@suse.cz>]

On Sat 12-07-14 21:06:45, Michael Kerrisk (man-pages) wrote:
> Late follow up on this thread..., since another question occurred in
> discussions with Jake.
> 
> On Fri, Apr 4, 2014 at 2:43 PM, Jan Kara <jack@suse.cz> wrote:
> > On Fri 04-04-14 09:35:50, Michael Kerrisk (man-pages) wrote:
> >> On 04/03/2014 10:52 PM, Jan Kara wrote:
> >> > On Thu 03-04-14 08:34:44, Michael Kerrisk (man-pages) wrote:
> [...]
> >> >>    Dealing with rename() events
> >> >>        The  IN_MOVED_FROM  and  IN_MOVED_TO events that are generated by
> >> >>        rename(2) are usually available as consecutive events when  read‐
> >> >>        ing from the inotify file descriptor.  However, this is not guar‐
> >> >>        anteed.  If multiple processes are triggering  events  for  moni‐
> >> >>        tored  objects,  then  (on rare occasions) an arbitrary number of
> >> >>        other events may appear between the IN_MOVED_FROM and IN_MOVED_TO
> >> >>        events.
> >> >>
> >> >>        Matching  up  the IN_MOVED_FROM and IN_MOVED_TO event pair gener‐
> >> >>        ated by rename(2) is thus inherently racy.  (Don't forget that if
> >> >>        an  object is renamed outside of a monitored directory, there may
> >> >>        not even be an IN_MOVED_TO event.)  Heuristic  approaches  (e.g.,
> >> >>        assume the events are always consecutive) can be used to ensure a
> >> >>        match in most cases, but will inevitably miss some cases, causing
> >> >>        the  application  to  perceive  the IN_MOVED_FROM and IN_MOVED_TO
> >> >>        events as being unrelated.  If watch  descriptors  are  destroyed
> >> >>        and  re-created as a result, then those watch descriptors will be
> >> >>        inconsistent with the watch descriptors in  any  pending  events.
> >> >>        (Re-creating the inotify file descriptor and rebuilding the cache
> >> >>        may be useful to deal with this scenario.)
> >> >   Well, but there's 'cookie' value meant exactly for matching up
> >> > IN_MOVED_FROM and IN_MOVED_TO events. And 'cookie' is guaranteed to be
> >> > unique at least within the inotify instance (in fact currently it is unique
> >> > within the whole system but I don't think we want to give that promise).
> >>
> >> Yes, that's already assumed by my discussion above (its described elsewhere
> >> in the page). But your comment makes me think I should add a few words to
> >> remind the reader of that fact. I'll do that.
> >   Yes, that would be good.
> >
> >> But, the point is that even with the cookie, matching the events is
> >> nontrivial, since:
> >>
> >> * There may not even be an IN_MOVED_FROM event
> >> * There may be an arbitrary number of other events in between the
> >>   IN_MOVED_FROM and the IN_MOVED_TO.
> >>
> >> Therefore, one has to use heuristic approaches such as "allow at least
> >> N millisconds" or "check the next N events" to see if there is an
> >> IN_MOVED_FROM that matches the IN_MOVED_TO. I can't see any way around
> >> that being inherently racy. (It's unfortunate that the kernel can't
> >> provide a guarantee that the two events are always consecutive, since
> >> that would simply user space's life considerably.)
> >   Yeah, it's unpleasant but doing that would be quite costly/complex at the
> > kernel side. And the race would in the worst case lead to application
> > thinking there's been file moved outside of watched area & a file moved
> > somewhere else inside the watched area. So the application will have to
> > possibly inspect that file. That doesn't seem too bad.
> 
> One further question. The IN_MOVED_FROM+IN_MOVED_TO pair may not be
> guaranteed to be contiguous in the read buffer, but is their insertion
> in the event queue guaranteed to be atomic from a user-space point of
> view? That is to say: having read an IN_MOVED_FROM event, does user
> space have the guarantee that if there is an IN_MOVED_TO event, then
> it will already be in the queue? The reason I ask is that this would
> affect how user space might try to read the IN_MOVED_TO event. If
> there is no such guarantee, then a read() (or select()/poll()) with
> (small) timeout is needed. If such a guarantee is provided, then a
> nonblocking read() would suffice.
  That's a good question... So the events are not generated atomically even
from userspace POV - i.e., a userspace process may see a state where
IN_MOVED_FROM event is already in the buffer but IN_MOVED_TO event isn't
generated yet.

> PS I just now found this code by John McCutchan
> https://git.gnome.org/browse/gnome-vfs/tree/modules/inotify-kernel.c#n570
> which suggests that the insertion of the event pair is not atomic
> w.r.t. user space. Still, I wonder if there is any definitive
> statement about this.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR