About I/O callbacks ...

[Davide Libenzi]

I was just thinking to implement I/O callbacks inside the kernel and test which
kind of performance could result compared to a select()/poll() implementation.
I prefer IO callbacks against async-io coz in this way is more direct to
implement an I/O driven state machine + coroutines.
This is a first draft :

#define ASCB_INFO_NPARAMS 4

struct ascb_info {
        unsigned long param[ASCB_INFO_NPARAMS];
};

struct iocb {
        struct iocb * next;
        struct task_struct * task;
        unsigned long event;
        int (*iocb_func)(struct ascb_info *, void *);
        void * data;
};

struct ascb {
        struct ascb * next;
        int (*ascb_func)(struct ascb_info *, void *);
        void * data;
        struct ascb_info info;
};

struct task_struct {
        ...
        struct ascb * ascb_list;
        spinlock_t ascb_list_lock;
        ...
};

struct file {
        ...
        struct iocb * iocb_list;
        spinlock_t iocb_list_lock;
        ...
};

The user call some user-space api to add the callback to the fd and this will
result ( inside the kernel ) to a call to :

int iocb_file_add(struct file * file,
                int (*iocb_func)(struct ascb_info *, void *), void * data,
                unsigned long event) {

/*
 *  Add the callback to the file list with task = current
 */

}

This is used to add callbacks to the task's list :

int ascb_task_add(struct task_struct * task,
                int (*ascb_func)(struct ascb_info *, void *),
                void * data, struct ascb_info * info) {

/*
 *  Add the callback to the task list
 */

}

Low level I/O layers will call this to dispatch I/O events :

int iocb_file_dispatch(struct file * file, unsigned long event) {

/*
 *  Scan the iocb_list and ( if event match ) call ascb_task_add()
 *  and remove ( y/n ? ) the callback from iocb_list
 */

}

In entry.S we'll have a call like do_signal() that will build the frame for a
callback call ( like do_signal() ) and will remove the entry from the list.
My first implementation should address only sockets but once the concept of
async callbacks is inside the task_struct this could be gradually extended to
std files and even used as an extension of signals.

Comments ?

- Davide