https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241808
Vincenzo Maffione <vmaffi...@freebsd.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|New |In Progress
--- Comment #1 from Vincenzo Maffione <vmaffi...@freebsd.org> ---
Very interesting, thanks for catching it.
I think the real problem here (even before r354288) is that nothing prevents a
call to kevent(fd, EV_ENABLE) to happen before kevent(fd, EVADD) is called, as
you are reporting.
On mevent_add() and mevent_update(), mevent_notify() is called to wakeup the
I/O thread, that will call kevent(changelist) to update the kernel.
A race condition is possible where the client calls mevent_add() and
mevent_update(EV_ENABLE) before the I/O thread has the chance to wake up and
calls mevent_build()+kevent(changelist), which is exactly what happens in your
case.
r354288 only makes this race condition more likely (this explains why I did not
catch the bug on my machine when I tested e1000).
Your fix makes the race condition less likely, but I think it's still there.
I tried to rework and simplify mevent.c to make sure EV_ADD is always called
before EV_ENABLE or EV_DISABLE.
Let me know what you think about that:
https://reviews.freebsd.org/D22286
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