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@xxxxxxx> 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@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
