[PATCH] posix message queues: send notifications via netlink

From: Manfred Spraul <manfred@colorfullife.com>

SIGEV_THREAD means that a given callback should be called in the context on a
new thread.  This must be done by the C library.  The kernel must deliver a
notice of the event to the C library when the callback should be called.

This patch switches to a new, simpler interface: User space creates a socket
with socket(PF_NETLINK, SOCK_RAW,0) and passes the fd to the mq_notify call
together with a cookie.  When the mq_notify() condition is satisfied, the
kernel "writes" the cookie to the socket.  User space then reads the cookie
and calls the appropriate callback.

include/linux/mqueue.h

@@ -30,8 +30,24 @@ struct mq_attr {
 	long	__reserved[4];	/* ignored for input, zeroed for output */
 };
 
+/*
+ * SIGEV_THREAD implementation:
+ * SIGEV_THREAD must be implemented in user space. If SIGEV_THREAD is passed
+ * to mq_notify, then
+ * - sigev_signo must be the file descriptor of an AF_NETLINK socket. It's not
+ *   necessary that the socket is bound.
+ * - sigev_value.sival_ptr must point to a cookie that is NOTIFY_COOKIE_LEN
+ *   bytes long.
+ * If the notification is triggered, then the cookie is sent to the netlink
+ * socket. The last byte of the cookie is replaced with the NOTIFY_?? codes:
+ * NOTIFY_WOKENUP if the notification got triggered, NOTIFY_REMOVED if it was
+ * removed, either due to a close() on the message queue fd or due to a
+ * mq_notify() that removed the notification.
+ */
 #define NOTIFY_NONE	0
 #define NOTIFY_WOKENUP	1
 #define NOTIFY_REMOVED	2
 
+#define NOTIFY_COOKIE_LEN	32
+
 #endif