Re: inotify_rm_watch() user-space safety requirements?

[Heinrich Schuchardt]

On 27.05.2014 19:25, Jeff Smith wrote:
> inotify's behavior concerning events from removed watches (they do
> happen) and watch descriptor reuse (beyond my knowledge) is currently
> undocumented.
>
> Although it mimics a standard multiplexing interface in most regards,
> writing a robust user-space handler is comparatively more complex due
> to the atypical delivery of "stale" wd events preceding an IN_IGNORE
> event and a lack of guarantees about how quickly a wd can be reused
> via inotify_add_watch(). Not being familiar with inotify/fsnotify
> internals, it's not trivially obvious to me how the fsnotify_group
> management is being done. Up to the present, I've maintained queues of
> "dead" wd wrappers (or at least a counter) to filter stale events, but
> I am clueless whether or not this is overkill.
>
> If removed descriptors are reserved until the IN_IGNORE event is
> drained from the read queue, could that be formally guaranteed? If
> it's not, is it functionality that could ever reasonably be expected
> to be added, short of some other form of new (optional?)
> queue-filter-on-rm functionality? It's my experience that the
> asynchronous handling of watch removals is a cost that seldom serves
> much user benefit.
> Regards,
> Jeff

Hello Jeff,

I tried to dive a bit into the code. This is what I understand:

Function inotify_ignored_and_remove_idr is called after the mark has 
been removed. This function puts an IN_IGNORED event onto the inotify 
queue and removes the watch descriptor from the list of used watch 
descriptors using function idr_remove.

With a test program I could receive the IN_IGNORED event. This behavior 
is currently not documented in the manpages (inotify.7 and 
inotify_rm_watch.2).

When inotify_add_watch is called it uses function idr_alloc_cyclic to 
assign a watch descriptor ID. This function starts looking for an unused 
id starting with the id after the last assigned watch descriptor.

This implies that in most cases inotify_add_watch will return a watch 
descriptor different to the one released by a prior call to 
inotify_rm_watch. But there is no guarantee.

I consider this a bug.

I CCed the maintainers of the inotify interface hoping that they can 
provide a better solution.

Until such a solution is provided I suggest you use the following 
workaround. After calling inotify_rm_watch read from the inotify file 
descriptor until you reach the matching IN_IGNORED event.

Only thereafter you can safely call inotify_add_watch again.

Best regards

Heinrich Schuchardt

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