On 04/07/2014 11:31 AM, Jan Kara wrote: > On Sun 06-04-14 11:00:29, Michael Kerrisk (man-pages) wrote: >> 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. > I had a look into that code again and it's all designed around the fact > that there's a single inode to notify. If you liked to have atomic rename > notifications, you'd have to rewrite that to work with two inodes, finding > out whether these two inodes are actually watched by the same group or > not... Doable but complex. Alternatively you could just lock down the whole > notification subsystem while generating rename events. But that's rather > costly. Just that we have the complications written down somewhere in case > someone wants to look into this in future. > >>> 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. Sorry for the late follow-up.... > But if I understand it right, you loose only the information for recreated > watches. So you effectively loose all the information about what has > happened inside the subtree of moved directory (or what has happened with > the moved file). But since you think it's a file / dir moved from outside > of watched area, you have to fully rescan that file / dir anyway. Ack on you summary there. > Sure > that's costly but if your heuristics for detecting rename works 99.9% of > time it should be OK, shouldn't it? And you have to have that code handling > caching file / dir written anyway for handling real moves from outside of > watched hierarchy. And ack on that. > Don't get me wrong, I understand it would be easier for userspace to get > atomic rename notifications, I'm just trying to understand what exactly is > painful so that I can compare the cost at the kernel side with the cost at > the userspace side... Yes, I was probably a little too strong in my statement. My perspective is that I'd tried to write an (experimental) application that would track *all* events for a file tree (modulo queue overflow), and then I encountered the wall of "rename() events are not consecutive", which basically rendered that task impossible because of the races involved. All that you say above also fits with my understanding. I was just (perhaps overly) disappointed to find that I couldn't (perfectly) achieve the tracking task that I'd attempted. (And furthermore, of course, the code became a bit more complicated to handle the possibility that some queued events may be for watch descriptors that are no longer valid.) Thanks for your response. 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
