Date: Fri, 15 Mar 2013 13:19:19 -0400 From: Ryan Stone <rysto32@gmail.com> To: Sebastian Huber <sebastian.huber@embedded-brains.de> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Purpose of kqueue_task? Message-ID: <CAFMmRNxbYpavy1XDuPYP--dZ57ifTZqC=PDBS%2Bv%2Bjx1KR0OrMw@mail.gmail.com> In-Reply-To: <51435063.7050907@embedded-brains.de> References: <51435063.7050907@embedded-brains.de>
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On Fri, Mar 15, 2013 at 12:46 PM, Sebastian Huber < sebastian.huber@embedded-brains.de> wrote: > Hello, > > I want to port the FreeBSD kqueue implementation to another operating > system (RTEMS in this case) to improve the Erlang support. > > I have difficulties to understand the purpose of the kqueue_task. This > function runs asynchronously. It obtains some locks and wakes up the > normal kqueue channel if (kq->kq_state & KQ_TASKDRAIN) == KQ_TASKDRAIN. > This state is only set in kqueue_close(). So most of the time the > kqueue_task only obtains some locks, clears a flag (KQ_TASKSCHED) and > releases the locks? > You missed the most important thing that it does: it calls KNOTE_LOCKED to wake up any waiters sleeping on this event. I suspect that it had to be done in a separate task due to lock ordering problems with the kq_global lock. The call to wakeup() is used to synchronize with kqueue_close to ensure that kqueue_close will not free the kqueue while the task is pending. Otherwise the task could run after kq was freed and crash the system.
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