Multiple locks and missing wakeup.

[Edward Tomasz Napierała]

Wiadomość napisana przez John Baldwin w dniu 8 kwi 2014, o godz. 20:12:
> 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.

Ah, now I get it.  Nifty!