Several unrelated beginner questions.

Several unrelated beginner questions.

[Konstantin Kowalski]

Hello everyone,

I am interested in Linux kernel programming (and OS kernels and 
general), and I am currently reading several books about Linux kernel. I 
have a few questions about it:


1.) Currently, I am reading 2 books about Linux kernel: Linux Device 
Drivers (3rd edition) and Linux Kernel Development (3rd edition).

I like both books and I am learning a lot from them.

I heard that both of this books are outdated, but so far all the 
information in this books seems valid and applicable. Is there better 
books you would recommend?

2.) In Linux Device Drivers, it states that module_exit(function) is 
discarded if module is built directly into kernel or if kernel is 
compiled with option to disallow loadable modules. But what if the 
module still has to do something during shutdown? Releasing memory is 
unimportant since it does not persist over reboot, but what if the 
module has to write something to a disk file, or do some other action?

3.) What's the deal with different kernel versions? I heard back in the 
2.x days, even kernels were stable and odd versions were experimental, 
but with 2.6 it changed.

So with 3.x kernels, are all of them experimental in the beginning and 
stable in the end? Also, with 3.x new versions seem to be released more 
often than in 2.1-2.5 days. Did the release cycle get smaller or is it 
just my imagination? Also, what does rc number mean?

4.) Currently, I am running linux-next, and it works great. Am I correct 
to assume that linux-next is supposed to have newest, shiniest and most 
unstable features? `uname -a` says that I am still running 3.8-next, but 
there is already 3.9 out. So which version is more experimental and 
least stable? Which one is the newest?

5.) How exactly does make/.config work? When I run `make oldconfig`, 
does it use the everything from the previous .config and only ask how to 
configure new features? And when I run `make` does it re-use old object 
files if nothing was changed in the specific file, or does it re-compile 
everything from scratch?

Thank you,

Kostyantyn Kovalskyy (Konstantin Kowalski)

Several unrelated beginner questions.

[Gaurav Jain]

Specifically regarding (3) and (4), please refer to this:
http://unixtravails.blogspot.ch/2012/07/linux-versioning-system-and-development.html

Best Regards
Gaurav Jain



On Thu, Mar 7, 2013 at 12:19 AM, Konstantin Kowalski <kostya-kow@mail.ru>wrote:

> Hello everyone,
>
> I am interested in Linux kernel programming (and OS kernels and
> general), and I am currently reading several books about Linux kernel. I
> have a few questions about it:
>
>
> 1.) Currently, I am reading 2 books about Linux kernel: Linux Device
> Drivers (3rd edition) and Linux Kernel Development (3rd edition).
>
> I like both books and I am learning a lot from them.
>
> I heard that both of this books are outdated, but so far all the
> information in this books seems valid and applicable. Is there better
> books you would recommend?
>
> 2.) In Linux Device Drivers, it states that module_exit(function) is
> discarded if module is built directly into kernel or if kernel is
> compiled with option to disallow loadable modules. But what if the
> module still has to do something during shutdown? Releasing memory is
> unimportant since it does not persist over reboot, but what if the
> module has to write something to a disk file, or do some other action?
>
> 3.) What's the deal with different kernel versions? I heard back in the
> 2.x days, even kernels were stable and odd versions were experimental,
> but with 2.6 it changed.
>
> So with 3.x kernels, are all of them experimental in the beginning and
> stable in the end? Also, with 3.x new versions seem to be released more
> often than in 2.1-2.5 days. Did the release cycle get smaller or is it
> just my imagination? Also, what does rc number mean?
>
> 4.) Currently, I am running linux-next, and it works great. Am I correct
> to assume that linux-next is supposed to have newest, shiniest and most
> unstable features? `uname -a` says that I am still running 3.8-next, but
> there is already 3.9 out. So which version is more experimental and
> least stable? Which one is the newest?
>
> 5.) How exactly does make/.config work? When I run `make oldconfig`,
> does it use the everything from the previous .config and only ask how to
> configure new features? And when I run `make` does it re-use old object
> files if nothing was changed in the specific file, or does it re-compile
> everything from scratch?
>
> Thank you,
>
> Kostyantyn Kovalskyy (Konstantin Kowalski)
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
Gaurav Jain
Associate Software Engineer
VxVM Escalations Team, SAMG
Symantec Software India Pvt. Ltd.
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Several unrelated beginner questions.

[Valdis.Kletnieks at vt.edu]

On Wed, 06 Mar 2013 18:19:09 -0500, Konstantin Kowalski said:

> 1.) Currently, I am reading 2 books about Linux kernel: Linux Device
> Drivers (3rd edition) and Linux Kernel Development (3rd edition).
>
> I like both books and I am learning a lot from them.
>
> I heard that both of this books are outdated, but so far all the
> information in this books seems valid and applicable. Is there better
> books you would recommend?

They're both still mostly applicable.  The concepts listed are still
valid - certain things need to be locked at certain times, things have
lifetimes, and so on.  The "outdated" is mostly places where the API has
changed slightly - for instance, where api_foo(struct bar *a, struct baz *b)
is now api_quux(struct bar *a, struct baz *b, int blat).  So you can't
cut-n-paste the code and expect it to still work.

> 2.) In Linux Device Drivers, it states that module_exit(function) is
> discarded if module is built directly into kernel or if kernel is
> compiled with option to disallow loadable modules. But what if the
> module still has to do something during shutdown? Releasing memory is
> unimportant since it does not persist over reboot, but what if the
> module has to write something to a disk file, or do some other action?

If your module has allocated 128M for a graphics buffer, you'll think
releasing memory is important. :)

Strictly speaking, a module *should* have already been quiesced and
taken care of business before module_exit() is called - there shouldn't
be much of anything left to do at that point.

(Hint - this is exactly the same question as "why is an empty ->release()
function considered a Bad Thing" - it's because release() and similar are
supposed to do the clean-up before the module exits)

> 3.) What's the deal with different kernel versions? I heard back in the
> 2.x days, even kernels were stable and odd versions were experimental,
> but with 2.6 it changed.

> So with 3.x kernels, are all of them experimental in the beginning and
> stable in the end? Also, with 3.x new versions seem to be released more
> often than in 2.1-2.5 days. Did the release cycle get smaller or is it
> just my imagination? Also, what does rc number mean?

The 3.x series is exactly the same policy as 2.6 was - Linus just decided
that 2.6.42 was too much and reset the counter, and he's been holding
to pretty close to every three months for releases for all that time.

And 2.1 got up to 2.1.142 or something insane like that in fewer years than it
took 2.6 to get to .42, so it isn't like releases are more frequent these days
:)

> 4.) Currently, I am running linux-next, and it works great. Am I correct

Lucky you.  I manage to break at least 2-3 things in linux-next per release
cycle. ;)

> to assume that linux-next is supposed to have newest, shiniest and most
> unstable features? `uname -a` says that I am still running 3.8-next, but
> there is already 3.9 out. So which version is more experimental and
> least stable? Which one is the newest?

Do another pull of the linux-next tree, it will say you're on 3.9-rc1-next now.
And even when it said 3.8-next, that was already "3.8 plus all the patches
queued for 3.9".  Now that Linus's tree is at 3.9-rc1, (closing the merge
window for major additions for 3.9) people will be dumping 3.10 material into
the linux-next tree.

> 5.) How exactly does make/.config work? When I run `make oldconfig`,
> does it use the everything from the previous .config and only ask how to
> configure new features?

Yes, that's what *should* happen.

>                          And when I run `make` does it re-use old object
> files if nothing was changed in the specific file, or does it re-compile
> everything from scratch?

Try it and see. :)  Note that sometimes, an apparently innocuous config change
can result in the rebuild of lots of files.  This is because some commonly used
.h file has a #ifdef CONFIG_FOO in it - and when you change FOO, then everybody
that includes that .h (even indirectly) ends up rebuilding.

But in general, if you touch only 1 or 2 .c files and no widely used .h files,
you'll just have to rebuild those .c's if they're modules.  If they're kernel
builtins, there's another 10 or 12 things that have to happen, but it's still
a lot faster than a full rebuild.
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Several unrelated beginner questions.

[Prabhakar Lad]

Hi Konstantin,

On Thu, Mar 7, 2013 at 4:49 AM, Konstantin Kowalski <kostya-kow@mail.ru> wrote:
> Hello everyone,
>
> I am interested in Linux kernel programming (and OS kernels and
> general), and I am currently reading several books about Linux kernel. I
> have a few questions about it:
>
>
> 1.) Currently, I am reading 2 books about Linux kernel: Linux Device
> Drivers (3rd edition) and Linux Kernel Development (3rd edition).
>
> I like both books and I am learning a lot from them.
>
> I heard that both of this books are outdated, but so far all the
> information in this books seems valid and applicable. Is there better
> books you would recommend?
>
I think you go ahead with above books only.

> 2.) In Linux Device Drivers, it states that module_exit(function) is
> discarded if module is built directly into kernel or if kernel is
> compiled with option to disallow loadable modules. But what if the
> module still has to do something during shutdown? Releasing memory is
> unimportant since it does not persist over reboot, but what if the
> module has to write something to a disk file, or do some other action?
>
Linux is OS, If not that way there are alternatives, what you can do is
use register_reboot_notifier() call, which arranges to call code blocks
when sys_reboot() is called.

> 3.) What's the deal with different kernel versions? I heard back in the
> 2.x days, even kernels were stable and odd versions were experimental,
> but with 2.6 it changed.
>
'different kernel versions'  means after each release of Linux it has version.
 like as you mentioned 2.6 above. Currently the naming convention is as
follows the number was composed as "A.B.C", where the number A denoted
the kernel version, the number B denoted the major revision of the kernel, and
the number C indicated the minor revision of the kernel.

> So with 3.x kernels, are all of them experimental in the beginning and
> stable in the end? Also, with 3.x new versions seem to be released more
> often than in 2.1-2.5 days. Did the release cycle get smaller or is it
> just my imagination? Also, what does rc number mean?
>
Stable kernel release depends on Linus/Greg etc.. Which involves lots
of work for them :). No The release cycle is 2-3 months.

> 4.) Currently, I am running linux-next, and it works great. Am I correct
> to assume that linux-next is supposed to have newest, shiniest and most
> unstable features? `uname -a` says that I am still running 3.8-next, but
> there is already 3.9 out. So which version is more experimental and
> least stable? Which one is the newest?
>
The linux-next tree to serve as a place where patches aimed to be merged
during the next development cycle are gathered. The newest one is 3.9.rc1
which is the release candidate. The release candidate usually have patches for
bug fixes no new feature or drivers add up here.

> 5.) How exactly does make/.config work? When I run `make oldconfig`,
> does it use the everything from the previous .config and only ask how to
> configure new features? And when I run `make` does it re-use old object
> files if nothing was changed in the specific file, or does it re-compile
> everything from scratch?
>
when you do `make oldconfig' 'It reads the existing .config file and prompts
the user for options in the current kernel source that are not found
in the file.
This is useful when taking an existing configuration and moving it to
a new kernel.

make program is an intelligent utility and works based on the changes you do
in your source files. If you have four files main.cpp, hello.cpp,
factorial.cpp and functions.h.
Here all the remaining files are dependent on functions.h and main.cpp
is dependent on
hello.cpp and factorial.cpp. So if you make any change in functions.h
then make will
recompile all the source files to generate new object files. But if
you make any change main.cpp,
as this is not dependent of any other fil, then in this case only
main.cpp file will be recompiled
and hellp.cpp and factorial.cpp will not be recompiled.

While compiling a file, make checks its object file and compare the
time staps, if source file
has newer time stamp than object file then it will generate new object
file assuming that
source file has been changed.

Regards,
--Prabhakar Lad
http://in.linkedin.com/pub/prabhakar-lad/19/92b/955

> Thank you,
>
> Kostyantyn Kovalskyy (Konstantin Kowalski)
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies