Re: [Xen-devel] [tools/libxl]: Possible bug in ao_cancel

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Koushik Chakravarty writes ("[tools/libxl]: Possible bug in ao_cancel"):
> I spotted a possible bug in ao_cancel (libxl_event.c) and wanted to run it 
> through you.

Hi, sure.  (Sorry for the delay replying, I have been away.  And I'm
going away again for the whole of next week...)

> In the ao_cancel(), we mark the parent->cancelling = 1 so that subsequence 
> cancel calls don't get entertained and mess things up. However, in my view, 
> setting this should be after we check for "parent->cancellables".
> 
> This is because, if someones invokes libxl_ao_cancel(), while there are no 
> cancellables registered, then further calls to libxl_ao_cancel() should not 
> be rejected - as the first call actually didn't do anything.
> I hope I am making myself clear.

Yes, your question is clear, thanks.  I don't think you're right,
though.

After cancelling is set, libxl__ao_cancellable_register rejects any
further attempts to set up any cancellables.  So I think the situation
you describe (where the libxl_ao_cancel would do nothing but return
CANCELLED because cancelling was already set, but in fact there is a
cancellable which could be notified) cannot arise.

Thanks,
Ian.

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