Re: Things I wish I'd known about Inotify

[Michael Kerrisk (man-pages)]

On 07/14/2014 01:28 PM, Jan Kara wrote:
> 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  readâ
>>>>>>        ing from the inotify file descriptor.  However, this is not guarâ
>>>>>>        anteed.  If multiple processes are triggering  events  for  moniâ
>>>>>>        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 generâ
>>>>>>        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.

Thanks for the confirmation, Jan. I also did some user-space
experimentation that pretty much showed the insertion must be nonatomic.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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