Re: bottom half

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2015-06-19 11:45, Edgar Fuß wrote:
> > "Runs on kernel stack in kernel space" is not the same thing as the Linux
> > concept of bottom half. :-)
> I don't know what the Linux (or VMS or Windows) concept of "nottom half" is.
> I thought I knew what the BSD concept of kernel halves is.

I can't comment on Windows - I have no idea.
VMS uses a very different solution, so it don't make much sense to talk 
about that here (or at least the parts that I know, which might be 
outdated).

If I remember Linux right, they have a fixed list of registered bottom 
half handlers (which of course ran out of space a long time ago), and 
then through some tricks extended it to be more general, but in essence 
the bottom half is the part of the device driver that runs after an 
interrupt to complete an I/O request. The top half being also running in 
the kernel, but in the context of a process that does the I/O, and the 
top half blocks until the I/O completes. And the bottom half is the part 
the unblocks the top half again. And each driver has its own bottom and 
top halves. One major point of the bottom halves is that when running 
the bottom half, interrupts are not blocked. A device driver normally do 
only a minimal amount of work in the interrupt handler itself, and then 
defer the rest of the work to the bottom half code, which will run at 
some later time.

Of course, I could be remembering this all wrong, and it might be 
outdated as well. So take what I write with a grain of salt. Or rather, 
read up on it in a Linux book instead.

I would say the bottom half concept in Linux is close to the softint 
stuff in NetBSD. But I might be wrong on that one, as I don't remember 
all the details of that either right now.

And tghe text you refer to in the 4.4BSD book then obviously are not 
describing the same concept as the Linux bottom halves, as the Linux 
bottom halves do not handle the hardware interrupt itself, and they can 
be interrupted by anything.

	Johnny

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Johnny Billquist                  || "I'm on a bus
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