Brendan (btrotter@gmail.com) on 5/9/06 wrote:
>myself (-@-.com) on 5/9/06 wrote:
>>
>>You don't get freed from those things in a microkernel
>>either - those disadvantages are inherent to both models.
>>Nobody said that a monolithic kernel fixes everything
>>magically.
>
>If a device driver is running as a single-threaded process,
>why would it need any re-entrancy locking at all?

Why do you think that has anything to do with microkernels?

Monolithic kernels have threads. It would be entirely
possible - although pretty stupid - to do a single driver
thread on a monolithic kernel, and have a simply queueing
model for delivering interrupts and requests to that driver.

Now, basically nobody would be as stupid as to do
this on a monolithic kernel. It's a pretty broken model,
and since the only reason to do it is to do something that
is actually pretty simple anyway (locking is very simple
if you just want to have total mutual exclusion), there's
just no real upside.

The reason microkernels often have that model is that if
you don't have direct interrupt delivery, and if all the
events are from some message queue anyway, it's just a
very natural way to do things. You actually tend to need
to do extra work to get re-entrant services and interrupts
delivered directly.

Of course, if you want good performance, heaven
forbid, you actually want to get interrupts delivered
directly, and you want to actually look at the whole
message queue, not just the top entry, and suddenly you
find that microkernels are actually in your way of
doing things well (or that you actually need to know
about things like locking, because you need to look into
the message queue by hand).

In general, locking isn't the problem in drivers, the hw
is. Undocumented, buggy, and with strange interactions
("Oh, DMA doesn't work properly with that other chipset?").

(Admittedly, equally often the driver problem is all the
infrastructure, and the fact that you have about a zillion
drivers that use old and broken rules that it turns out
doesn't work really well when you have a thousand disks
directly connected. Oh, well).

Linus