signal/timer/event: KAIO eventfd support example

This is an example about how to add eventfd support to the current KAIO code,
in order to enable KAIO to post readiness events to a pollable fd (hence
compatible with POSIX select/poll).  The KAIO code simply signals the eventfd
fd when events are ready, and this triggers a POLLIN in the fd.  This patch
uses a reserved for future use member of the struct iocb to pass an eventfd
file descriptor, that KAIO will use to post events every time a request
completes.  At that point, an aio_getevents() will return the completed result
to a struct io_event.  I made a quick test program to verify the patch, and it
runs fine here:

http://www.xmailserver.org/eventfd-aio-test.c

The test program uses poll(2), but it'd, of course, work with select and epoll
too.

This can allow to schedule both block I/O and other poll-able devices
requests, and wait for results using select/poll/epoll.  In a typical
scenario, an application would submit KAIO request using aio_submit(), and
will also use epoll_ctl() on the whole other class of devices (that with the
addition of signals, timers and user events, now it's pretty much complete),
and then would:

	epoll_wait(...);
	for_each_event {
		if (curr_event_is_kaiofd) {
			aio_getevents();
			dispatch_aio_events();
		} else {
			dispatch_epoll_event();
		}
	}

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/aio_abi.h

@@ -45,6 +45,14 @@ enum {
 	IOCB_CMD_PWRITEV = 8,
 };
 
+/*
+ * Valid flags for the "aio_flags" member of the "struct iocb".
+ *
+ * IOCB_FLAG_RESFD - Set if the "aio_resfd" member of the "struct iocb"
+ *                   is valid.
+ */
+#define IOCB_FLAG_RESFD		(1 << 0)
+
 /* read() from /dev/aio returns these structures. */
 struct io_event {
 	__u64		data;		/* the data field from the iocb */
@@ -84,7 +92,15 @@ struct iocb {
 
 	/* extra parameters */
 	__u64	aio_reserved2;	/* TODO: use this for a (struct sigevent *) */
-	__u64	aio_reserved3;
+
+	/* flags for the "struct iocb" */
+	__u32	aio_flags;
+
+	/*
+	 * if the IOCB_FLAG_RESFD flag of "aio_flags" is set, this is an
+	 * eventfd to signal AIO readiness to
+	 */
+	__u32	aio_resfd;
 }; /* 64 bytes */
 
 #undef IFBIG