Date: Tue, 8 Apr 2014 18:45:49 +0200 From: =?iso-8859-2?Q?Edward_Tomasz_Napiera=B3a?= <trasz@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org, hackers@freebsd.org Subject: Re: Multiple locks and missing wakeup. Message-ID: <F18FB5F4-52AE-4787-AF6B-D0111CFFA844@freebsd.org> In-Reply-To: <201404081001.31219.jhb@freebsd.org> References: <0D69A6A8-43D1-41FB-8C2D-00F5CAD9C86E@FreeBSD.org> <201404081001.31219.jhb@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
Wiadomo=B6=E6 napisana przez John Baldwin w dniu 8 kwi 2014, o godz. =
16:01:
> On Tuesday, April 08, 2014 2:34:30 am Edward Tomasz Napiera=B3a wrote:
>> Let's say I have a kernel thread processing elements from a queue,
>> sleeping until there is work to do; something like this:
>>=20
>> mtx_lock(&mtx1);
>> for (;;) {
>> while (!LIST_EMPTY(&list1)) {
>> elt =3D LIST_FIRST(&list1);
>> do_stuff(elt);
>> LIST_REMOVE(&list1, elt);
>> }
>> sleep(&list1, &mtx1);
>> }
>> mtx_unlock(&mtx1);
>>=20
>> 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:
>=20
> 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.
[..]
> 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?
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?F18FB5F4-52AE-4787-AF6B-D0111CFFA844>