Re: [Qemu-devel] [PATCH 2/3] usb-mtp: Add support for inotify based file monitoring

[Bandan Das <bsd@redhat.com>]

Bandan Das <bsd@redhat.com> writes:

> Gerd Hoffmann <kraxel@redhat.com> writes:
>
>> On Di, 2015-11-03 at 19:00 -0500, Bandan Das wrote:
>>> +                    /* Add a new watch asap so as to not lose events
>>> */
>>
>> This comment sounds like there is a race ("asap").  There isn't one,
>> correct ordering (adding the watch before reading the directory) is
>
> Hmm, seems like there's still a small window. We may not have even
> started processing the event because we are still processing the earlier
> ones.
>
>> enough to make sure you don't miss anything.  You might see create
>> events for objects already in the tree though, are you prepared to
>> handle that?
>
> Oh, interesting.  Current version will happily add duplicate entries.
> I will add a check.

By the way, did you mean this as a duplicate create event ?
I took a quick look at fs/notify/inotify_fsnotify.c:

int inotify_handle_event(...
    ret = fsnotify_add_event(group, fsn_event, inotify_merge);                                                                                                              
        if (ret) {
                /* Our event wasn't used in the end. Free it. */
                fsnotify_destroy_event(group, fsn_event);
        }

So, atleast for consecutive duplicate events, the kernel seems to be doing
some filtering of its own.

>> cheers,
>>   Gerd