aio completion notification

Summary:
-------

  The current 2.6 kernel does not support notification of user space via
an RT signal upon an asynchronous IO completion. The POSIX specification
states that when an AIO request completes, a signal can be delivered to
the application as notification.

  The aioevent patch adds a struct sigevent *aio_sigeventp to the iocb.
The relevant fields (pid, signal number and value) are stored in the kiocb
for use when the request completes.

  That sigevent structure is filled by the application as part of the AIO
request preparation. Upon request completion, the kernel notifies the
application using those sigevent parameters. If SIGEV_NONE has been specified,
then the old behaviour is retained and the application must rely on polling
the completion queue using io_getevents().

Details:
-------

  A struct sigevent *aio_sigeventp is added to struct iocb in
include/linux/aio_abi.h

  An enum {IO_NOTIFY_SIGNAL = 0, IO_NOTIFY_THREAD_ID = 1} is added in
include/linux/aio.h:

	- IO_NOTIFY_SIGNAL means that the signal is to be sent to the
	  requesting thread 

	- IO_NOTIFY_THREAD_ID means that the signal is to be sent to a
	  specifi thread.

  The following fields are added to struct kiocb in include/linux/aio.h:

	- pid_t ki_pid: target of the signal

	- __u16 ki_signo: signal number

	- __u16 ki_notify: kind of notification, IO_NOTIFY_SIGNAL or
			   IO_NOTIFY_THREAD_ID

	- uid_t ki_uid, ki_euid: filled with the submitter credentials

	- sigval_t ki_sigev_value: value stuffed in siginfo

  these fields are only valid if ki_signo != 0.

	

  In io_submit_one(), if the application provided a sigevent then
iocb_setup_sigevent() is called which does the following:

	- save current->uid and current->euid in the kiocb fields ki_uid and
	  ki_euid for use in the completion path to check permissions

	- check access to the user sigevent

	- extract the needed fields from the sigevent (pid, signo, and value).
	  If the signal number passed from userspace is 0 then no notification
	  is to occur and ki_signo is set to 0

	- check whether the submitting thread wants to be notified directly
	  (sigevent->sigev_notify_thread_id is 0) or wants the signal to be sent
	  to another thread.
	  In the latter case a check is made to assert that the target thread
	  is in the same thread group

	- fill in the kiocb fields (ki_pid, ki_signo, ki_notify and ki_sigev_value)
	  for that request.

  Upon request completion, in aio_complete(), if ki_signo is not 0, then
__aio_send_signal() is called which sends the signal as follows:

	- fill in the siginfo struct to be sent to the application

	- check whether we have permission to signal the given thread

	- send the signal