Date: Tue, 8 Apr 2014 14:12:10 -0400 From: John Baldwin <jhb@freebsd.org> To: Edward Tomasz =?iso-8859-2?q?Napiera=B3a?= <trasz@freebsd.org> Cc: freebsd-hackers@freebsd.org, hackers@freebsd.org Subject: Re: Multiple locks and missing wakeup. Message-ID: <201404081412.10066.jhb@freebsd.org> In-Reply-To: <F18FB5F4-52AE-4787-AF6B-D0111CFFA844@freebsd.org> References: <0D69A6A8-43D1-41FB-8C2D-00F5CAD9C86E@FreeBSD.org> <201404081001.31219.jhb@freebsd.org> <F18FB5F4-52AE-4787-AF6B-D0111CFFA844@freebsd.org>
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On Tuesday, April 08, 2014 12:45:49 pm Edward Tomasz Napierała wrote:
> Wiadomość napisana przez John Baldwin w dniu 8 kwi 2014, o godz. 16:01:
>
> > On Tuesday, April 08, 2014 2:34:30 am Edward Tomasz Napierała wrote:
> >> Let's say I have a kernel thread processing elements from a queue,
> >> sleeping until there is work to do; something like this:
> >>
> >> mtx_lock(&mtx1);
> >> for (;;) {
> >> while (!LIST_EMPTY(&list1)) {
> >> elt = LIST_FIRST(&list1);
> >> do_stuff(elt);
> >> LIST_REMOVE(&list1, elt);
> >> }
> >> sleep(&list1, &mtx1);
> >> }
> >> mtx_unlock(&mtx1);
> >>
> >> Now, is there some way to make it work with two lists, protected
> >> by different mutexes? The mutex part is crucial here; the whole
> >> point of this is to reduce lock contention on one of the lists. The
> >> following code would result in a missing wakeup:
> >
> > All our sleep primitives in the kernel only accept a single wait channel.
> > It sounds like you want something more like select() or poll() where you
> > can specify multiple wait channels. There isn't a good way to do that
> > currently. You could write one, but it would be a bit hard to do
> > correctly.
>
> Perhaps I should have been more clear: I'm ok with a single wait
> channel. The problem is that there is no way to pass more than one
> mutex to the sleep() function, so we can miss wakeup for the list
> protected by the second lock, if something gets enqueued between
> releasing mtx2 and calling sleep().
>
> > In practice you'd end up implementing something that boiled
> > down to having a single wait channel with a common lock that protected
> > it so you could do something like:
>
> The whole purpose of this is to avoid locking mtx1 in the the enqueue
> routine for the second list, for contention reasons.
Ah, but note that I didn't lock mtx1 in the enqueue routine, I marked
the 'combo_mtx' which is only used for the sleep/wakeup.
> [..]
>
> > Another way to do this would be to be a bit more poll-like (e.g. if
> > you wanted a generic mechanism for this) where you have some sort of
> > 'poller' structure and you set a flag before starting a scan of all
> > your backends. Any wakeup that occurs while scanning clears the
> > flag, and you only sleep if the flag is still set at the end of the
> > scan, etc.
>
> But the flag would have to be protected by the mutex we pass
> to sleep(), and would require grabbing that mutex in both enqueue
> routines, right?
Yep.
--
John Baldwin
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