How to best overload the fileops ?

[John-Mark Gurney]

Yuri wrote this message on Wed, Aug 21, 2013 at 11:37 -0700:
> I am working on linux epoll functionality for linuxlator. It implements 
> epoll through kqueue, but there is the need to overload fo_close 
> function in fileops to free some memory.

How did this memory get allocated in the first place?  Why does it need
to be free'd in fo_close and not another location?

> There is no generic mechanism defined in the kernel sources allowing to 
> do this, and it isn't desirable to do this in a hackish way. So I am 
> suggesting this particular way, see code snippets below.
> This approach is inspired by how C++ classes are sub-classed, with C++ 
> class being similar to kernel file descriptor type and C++ vtbl being 
> similar to fileops.
> 
> I am looking for an opinion(s) on these questions:
> * Is such code is acceptable for kernel?
> * Does it look too ugly?
> * Any suggestions on how to improve it?
> 
> As the system develops, other places may require to do such overloading 
> too, so this approach can be reused.
> 
> Thank you,
> Yuri
> 
> 
> *** In sys/file.h add these macros (they define how overloading is done):
> #define FDCLASS_DEFINE(cls)                        \
>     struct fileops* fdcls_##cls##_fileops(void);            \
>     struct fileops* fdcls_##cls##_fileops(void) {            \
>         return (&cls##ops);                    \
>     }
> #define FDCLASS_INHERIT(cls, cls_parent, cls_init_func)            \
>     extern struct fileops* fdcls_##cls_parent##_fileops(void);    \
>     static void cls##_fdcls_init(void *dummy __unused) {        \
>         cls##ops = *fdcls_##cls_parent##_fileops();        \
>         cls_init_func();                    \
>     }                                \
>     SYSINIT(cls##_fdcls, SI_SUB_PSEUDO, SI_ORDER_ANY, cls##_fdcls_init, 
> NULL);
> 
> *** In the end of kern/kern_event.c add the line exposing kqueue's fileops:
> FDCLASS_DEFINE(kqueue)
> 
> *** In linux_epoll.c add code that would initialize epoll fileops with 
> the base class fileops:
> /* overload kqueue fileops */
> static struct fileops epollops;
> static struct fileops epollops_base;
> static void
> epoll_init(void) {
>     /* overload only fo_close operation */
>     epollops_base = epollops;
>     epollops.fo_close = epoll_close;
> }
> FDCLASS_INHERIT(epoll, kqueue, epoll_init)
> 
> 
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-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."