On 04/04/2014 02:43 PM, Jan Kara 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. Yep, I imagined that was probably the reason. > 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. It's actually very bad. See the text above. The point is that one likely treatment on an IN_MOVED_FROM event that has no IN_MOVED_TO is to remove the watches for the moved out subtree. If it turns out that this really was just a rename(), then on the IN_MOVED_TO, the watches will be recreated *with different watch descriptors*, thus invalidating the watch descriptors in any queued but as yet unprocessed inotify events. See what I mean? That's quite painful for user space. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
