Jan Hudec wrote:
On Mon, Dec 09, 2002 at 04:36:50PM +0100, Edward Gerhold wrote:
Hmmm. That 's new for me. In my Books ldd (german) on page 36
and lnx-networkarchitecture (german) on page 37 they write, we
have to use the MOD_INC_.. and MOD_DEC_.. macros to set MOD_IN_USE.
MOD_INC_USE_COUNT has to be used _before_ opening any devices.
MOD_DEC_USE_COUNT has to be used _after_ closing any devices.
(Wehrle writes: A dev->open() increases the usage-counter by 1,
a dev->close() decreases the counter by 1 - both automatically)
MOD_IN_USE is used to prevent from a module_cleanup when any devices
which need the module are busy. Having the device busy, but the
module removed, would cause segmentation faults or kernel panics.
MOD_IN_USE can be influenced via ioctl.
Now I don't understand why i shouldn't use the macros. I've read now
a few times that i'll have to (i remember also lkmpg gives the advice
to use them, the book linux-kernelprogramming (german) gives the advice,
too). Who can lead me out ?
Every code, where there is a reason to call MOD_INC/DEC_USE_COUNT, is
broken, because there is a race.
When there is a module M that provides some function (say F), the
count for M must be incremented _before_ the first instruction of F is
executed and decremented only _after_ the last instruction of F has
finished. Thus you have to make sure that anyone who calls F increments
the use count. For filesystems, devices and the like, there is always on
owner field somewhere that is used by generic code to increment use
count of module that implements the file/device/... For functions used
by other module by means of dynamick linking, the linker takes care of
that. Any other entry point has a problem, but there should not be any
possibility for other entry point. (Modules can't sanely add syscalls).
-------------------------------------------------------------------------------
Jan 'Bulb' Hudec <bulb@ucw.cz>
Yes.
I can use this with T.L. Madhus warning plus the other mails to the subject.
It also describes the same like in the books. I feel cleared up now. I'm out.
Thank you very much.
Edward
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
