knotes

[Kostik Belousov]

On Tue, Mar 03, 2009 at 02:49:45PM -0800, Matthew Fleming wrote:
> I am trying to understand the knote system (on 6.1) and I am having some
> troubles.
> 
> Specifically, I am confused by the uses of KN_DETACHED and EV_ONESHOT.
> >From what I can determine from the comments and code, knotes have a
> filterops member, kn_fop.  This among other things has a callback to
> handle when a note is attached and detached.
> 
> But e.g. in knlist_clear(), when knlist_remove_kq() removes a knote from
> the list, it sets KN_DETACHED but does not call the kn_fop->f_detach
> routine.  Then, in the killkn case, KN_DETACHED is set (again).
> Otherwise, EV_ONESHOT is set, presumably so that kqueue_scan() will run
> on the knote.  However, kqueue_scan() won't call kn_fop->f_detach either
> because KN_DETACHED is already set.
> 
> It seems that in knlist_cleardel(), the killkn case should be calling
> kn_fop->f_detach before knote_drop().  It also seems that the !killkn
> case should not have KN_DETACHED set, which means that
> knlist_remove_kq() can't set it.  Alternatively, knlist_remove_kq()
> should be calling kn_fop->f_detach itself before setting KN_DETACHED.
> But in that case I'm not sure I see why there needs to be a use of
> EV_ONESHOT.
> 
> So am I reading this wrong, understanding it wrong, or is there a bug in
> the code?

There are two pathes to each knote, one from the kqueue(2) syscall,
and another is from the kernel subsystem. My understanding is that
f_detach handler is intended to be called from the syscall path
only.

For instance, pipe destructor sys_pipe.c:pipeclose() calls knlist_clear,
i.e. knlist_cleardel with killkn == 0. The detach handler for the pipe
removes the knote from the corresponding pipe' knlist. It seems that
it is simply wrong to call pipe f_detach from knlist_clear().
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