man/man9/msgport.9

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.Dd September 17, 2012
.Dt MSGPORT 9
.Os
.Sh NAME
.Nm lwkt_initport_thread ,
.Nm lwkt_initport_spin ,
.Nm lwkt_initport_serialize ,
.Nm lwkt_initport_panic ,
.Nm lwkt_initport_replyonly_null ,
.Nm lwkt_initport_replyonly ,
.Nm lwkt_initport_putonly ,
.Nm lwkt_sendmsg ,
.Nm lwkt_domsg ,
.Nm lwkt_forwardmsg ,
.Nm lwkt_abortmsg ,
.Nm lwkt_initmsg ,
.Nm lwkt_initmsg_abortable ,
.Nm lwkt_beginmsg ,
.Nm lwkt_replymsg ,
.Nm lwkt_getport ,
.Nm lwkt_waitport ,
.Nm lwkt_waitmsg ,
.Nm lwkt_checkmsg ,
.Nm lwkt_dropmsg
.Nd LWKT message passing interface
.Sh SYNOPSIS
.In sys/msgport.h
.Ft void
.Fn lwkt_initport_thread "lwkt_port_t port" "struct thread *td"
.Ft void
.Fn lwkt_initport_spin "lwkt_port_t port" "struct thread *td"
.Ft void
.Fn lwkt_initport_serialize "lwkt_port_t port" "struct lwkt_serialize *slz"
.Ft void
.Fn lwkt_initport_panic "lwkt_port_t port"
.Ft void
.Fn lwkt_initport_replyonly_null "lwkt_port_t port"
.Ft void
.Fn lwkt_initport_replyonly "lwkt_port_t port" "void (*rportfn)(lwkt_port_t, lwkt_msg_t)"
.Ft void
.Fn lwkt_initport_putonly "lwkt_port_t port" "int (*pportfn)(lwkt_port_t, lwkt_msg_t)"
.Ft void
.Fn lwkt_sendmsg "lwkt_port_t port" "lwkt_msg_t msg"
.Ft int
.Fn lwkt_domsg "lwkt_port_t port" "lwkt_msg_t msg" "int flags"
.Ft int
.Fn lwkt_forwardmsg "lwkt_port_t port" "lwkt_msg_t msg"
.Ft void
.Fn lwkt_abortmsg "lwkt_msg_t msg"
.In sys/msgport2.h
.Ft void
.Fn lwkt_initmsg "lwkt_msg_t msg" "lwkt_port_t rport" "int flags"
.Ft void
.Fn lwkt_initmsg_abortable "lwkt_msg_t msg" "lwkt_port_t rport" "int flags" "void (*abortfn)(lwkt_msg_t)"
.Ft int
.Fn lwkt_beginmsg "lwkt_port_t port" "lwkt_msg_t msg"
.Ft void
.Fn lwkt_replymsg "lwkt_msg_t msg" "int error"
.Ft void *
.Fn lwkt_getport "lwkt_port_t port"
.Ft void *
.Fn lwkt_waitport "lwkt_port_t port" "int flags"
.Ft int
.Fn lwkt_waitmsg "lwkt_msg_t msg" "int flags"
.Ft int
.Fn lwkt_checkmsg "lwkt_msg_t msg"
.Ft void
.Fn lwkt_dropmsg "lwkt_msg_t msg"
.Sh DESCRIPTION
Light weight kernel threads in
.Dx
may use a message passing interface to communicate with each other.
Messages are sent to message ports.
All light weight kernel threads have a built-in message port, but you may create
additional ports if necessary.
The following types of message ports are available:
.Bl -bullet
.It
thread ports
.It
spin ports
.It
serializer ports
.El
.Pp
Ports of type
.Sq thread
are owned by a single light weight kernel thread.
When a message is sent to a port of type
.Sq thread ,
only the owner of that port is allowed to retrieve the message from it.
When a message is sent to a port of type
.Sq spin
or to a port of type
.Sq serializer ,
multiple threads are allowed to check that port for new
messages and compete to retrieve them.
You define the port type when you initialize the port.
By default, the built-in port of every light weight kernel thread is
automatically initialized to type
.Sq thread .
.Pp
When a message is sent, the receiver should normally send back a reply.
The reply is sent to the reply port that is registered on the original message.
Messages can be replied to synchronously or asynchronously.
The sender may request a synchronous or asynchronous reply to the message,
however the target port will ultimately decide how the message will be treated.
.Sh MESSAGE FUNCTIONS
Messages must be initialized before being used.
The
.Fn lwkt_initmsg
function initializes a message.
The
.Fa rport
argument identifies the reply port which will be used for asynchronous replies.
The
.Fa flags
argument sets any required flags for this message.
Flags passed this way will simply be or'ed to any already existing flags on the
message.
.Pp
The
.Fn lwkt_initmsg_abortable
function is similar to
.Fn lwkt_initmsg
but it takes an additional parameter
.Fa abortfn
which defines the abort function for this message.
.Pp
The
.Fn lwkt_sendmsg
function requests an asynchronous reply, sends the message and returns
immediately.
Under normal circumstances, users of this function may always expect the reply
to be queued to the reply port registered on the message.
The
.Fa port
argument defines the target port to which the
.Fa msg
message will be sent.
.Pp
The
.Fn lwkt_domsg
function requests a synchronous reply, sends the message and does not return
until the message has been replied to.
If the target port supports synchronous reply, this function will return that
reply immediately.
If not, and this is the most common case, this function will block and wait for the
reply to arrive and then return it.
The
.Fa port
argument defines the target port to which the
.Fa msg
message will be sent.
.Pp
The
.Fn lwkt_replymsg
function replies to a message that was processed asynchronously by the target
port.
This function is used by the thread on the receiving side.
The
.Fa msg
argument is the message being replied to and the
.Fa error
argument is the actual response to send back.
.Pp
The
.Fn lwkt_forwardmsg
simply forwards a message to another port.
The
.Fa port
argument defines the target port to which the
.Fa msg
message will be sent.
.Pp
If a message has been initialized as abortable, you can use the
.Fn lwkt_abortmsg
function to try to abort it.
The
.Fa abortfn
passed upon the initialisation with
.Fn lwkt_initmsg_abortable
will be called by this function.
.Pp
The
.Fn lwkt_dropmsg
will dequeue the specified message from the target port it was sent to and makes
it look like it was never sent.
This function can only be used by the thread that owns the target port.
.Sh PORT FUNCTIONS
The
.Fn lwkt_initport_thread
initializes the specified
.Fa port
with the default
.Sq thread
port type handlers.
The
.Fa td
argument defines the owner thread of the port and only that thread is allowed to
receive messages on it.
.Pp
The
.Fn lwkt_initport_spin
initializes the specified
.Fa port
with the default
.Sq spin
port type handlers.
The
.Fa td
argument defines the owner thread of the port, for cases where thread built-in
ports are initialized as
.Sq spin
ports.
If
.Dv NULL
is passed, then the port will not have a defined owner, so functions like
.Fn lwkt_dropmsg
will not be available for this port.
This function will also initialize the embedded spinlock within the
.Vt lwkt_port
structure which will protect subsequent port access.
.Pp
The
.Fn lwkt_initport_serialize
function initializes the specified
.Fa port
with the default
.Sq serializer
port type handlers.
The subsequent port access will be protected by the passed
.Fa slz
serializer lock.
.Pp
The
.Fn lwkt_getport
function checks the specified
.Fa port
for available messages, dequeues the first one and returns it.
If no messages are available then
.Dv NULL
is returned instead.
This function is used by threads on the receiving side.
.Pp
The
.Fn lwkt_waitport
function checks the specified
.Fa port
for available messages, dequeues the first one and returns it.
If no messages are available then the caller thread will sleep until a message
arrives on the specified port.
The
.Fa flags
argument defines the flags used for the sleep.
This function is used by threads on the receiving side.
.Sh SPECIAL PORT INITIALIZERS
The
.Fn lwkt_initport_replyonly
function initializes a
.Fa port
which is used only as reply port and may have a custom reply port handler.
The reply port handler is specified with the
.Fa rportfn
argument.
All the other handlers will panic the system if they are called.
This initializer is normally used on ports for freeing resources after the
messages have fulfilled their purpose.
.Pp
The
.Fn lwkt_initport_replyonly_null
function initializes a
.Fa port
which is used only as reply port.
The reply port handler will simply mark the message as being done and will not
attempt to queue it.
All the other handlers will panic the system if they are called.
.Pp
The
.Fn lwkt_initport_putonly
function initializes a
.Fa port
which is used only as target port.
The putport handler is specified with the
.Fa pportfn
argument.
All the other handlers will panic the system if they are called.
.Pp
The
.Fn lwkt_initport_panic
function initializes a
.Fa port
which will panic the system if any of its handlers are called.
This function is sometimes used to initialize a reply-only port which does not
expect the messages to be replied to, e.g.\& when the messages should be
consumed by the receiving thread and never replied back.
.Sh INTERNAL MESSAGE FUNCTIONS
The following functions are used only by the infrastructure, you should not
need to use them directly unless in very rare cases.
.Pp
The
.Fn lwkt_beginmsg
function simply calls the target port's putport handler.
This function is only called by the
.Fn lwkt_sendmsg
and
.Fn lwkt_replymsg
functions.
The putport handler returns
.Er EASYNC
for messages processed asynchronously or any other value for messages processed
synchronously.
That return value of the putport handler is propagated by this function.
The
.Fa port
argument defines the target port to which the
.Fa msg
message will be sent.
.Pp
The
.Fn lwkt_waitmsg
function puts the caller to sleep until the specified
.Fa msg
message has been replied to.
The
.Fa flags
argument defines the flags used for the sleep.
.Sh FILES
The LWKT msgport implementation resides in
.Pa sys/kern/lwkt_msgport.c .
.Sh EXAMPLES
.Bd -literal
/*
 * Example 1: per CPU threads.
 *
 */

#include <sys/thread.h>
#include <sys/msgport.h>
#include <sys/msgport2.h>

static void my_service_loop(void *dummy);
lwkt_port_t my_service_portfn(int cpu);
void my_service_sendmsg(lwkt_msg_t lmsg, int cpu);
int my_service_domsg(lwkt_msg_t lmsg, int cpu);

/* Array of per-CPU target ports */
struct lwkt_port *my_service_ports[MAXCPU];

/*
 * Create per-cpu threads for handling msg processing.  Remember that built-in
 * lwkt ports are automatically initialized to type 'thread' so we don't need
 * to initialize them explicitly.
 */
static void
my_per_cpu_service_init(void)
{
	int i;
	thread_t td;

	for (i = 0; i < ncpus; ++i) {
		lwkt_create(my_service_loop, NULL, &td,
			    NULL, 0, i, "myservice_cpu %d", i);
		my_service_ports[i] = &td->td_msgport;
	}
}

/*
 * This is the routine executed by the service threads on each CPU.
 */
static void
my_service_loop(void *dummy __unused)
{
	lwkt_msg_t msg;
	thread_t td = curthread;
	int cpu = curthread->td_gd->gd_cpuid;

	while ((msg = lwkt_waitport(&td->td_msgport, 0)) != NULL) {
		/* Do some work in the receiver thread context. */
		kprintf("Received message on CPU %d.\en", cpu);

		/* And finally reply to the message. */
		lwkt_replymsg(msg, 0);
	}
}

/*
 * Given a CPU id, return our respective service port.
 */
__inline lwkt_port_t
my_service_portfn(int cpu)
{
	return my_service_ports[cpu];
}

/*
 * Send an asynchronous message to the service thread on a specific CPU.
 */
void
my_service_sendmsg(lwkt_msg_t lmsg, int cpu)
{
	KKASSERT(cpu < ncpus);
	lwkt_sendmsg(my_service_portfn(cpu), lmsg);
}

/*
 * Send a synchronous message to the service thread on a specific CPU.
 */
int
my_service_domsg(lwkt_msg_t lmsg, int cpu)
{
	KKASSERT(cpu < ncpus);
	return lwkt_domsg(my_service_portfn(cpu), lmsg, 0);
}

/*
 * Example use case. Initialize the service threads and send each one a
 * message.
 */
static void
mod_load(void)
{
	lwkt_msg	lmsg;
	lwkt_port_t	builtin_port = &curthread->td_msgport;
	int		i;

	my_per_cpu_service_init();
	for (i=0; i<ncpus; ++i) {
		kprintf("Sending msg to CPU %d.\en", i);
		lwkt_initmsg(&lmsg, builtin_port, 0);
		my_service_domsg(&lmsg, i);
	}
}

/*
 * Example 2: Dynamic allocated message passing with automatic free.
 *
 * This scenario is used when resources need to be freed after the message
 * has been replied to. Features:
 * - An argument is passed within the message.
 * - Messages are allocated with kmalloc(). Replying to the msg, kfree()s it.
 */

#include <sys/thread.h>
#include <sys/msgport.h>
#include <sys/msgport2.h>

void my_service_queue(void *arg);

lwkt_port my_autofree_rport;
lwkt_port_t my_service_port;

/*
 * Use this function to send messages with a void * argument to our
 * service thread.
 */
void
my_service_queue(void *arg)
{
	lwkt_msg_t msg;

	msg = kmalloc(sizeof(*msg), M_TEMP, M_WAITOK);

	/* Set reply port to autofree. */
	lwkt_initmsg(msg, &my_autofree_rport, 0);

	/* Attach the argument to the message. */
	msg->u.ms_resultp = arg;

	/* Send it. */
	lwkt_sendmsg(my_service_port, msg);
}

/*
 * This is the routine executed by our service thread.
 */
static void
my_service_loop(void *dummy __unused)
{
	lwkt_msg_t msg;
	thread_t td = curthread;

	while ((msg = lwkt_waitport(&td->td_msgport, 0)) != NULL) {
		/*
		 * Do some work in the receiver thread context.  In this
		 * example, the sender wrote his name in the argument he
		 * sent us.  We print it here.
		 */
		char *arg = msg->u.ms_resultp;
		kprintf("%s: Hi %s! Got your msg.\en", curthread->td_comm, arg);

		/* And finally reply to the message. */
		lwkt_replymsg(msg, 0);
	}
}

static void
my_autofree_reply(lwkt_port_t port, lwkt_msg_t msg)
{
	kfree(msg->u.ms_resultp, M_TEMP);
	kfree(msg, M_TEMP);
}

static void
my_service_init(void)
{
	thread_t tdp;

	/* Initialize our auto free reply port. */
	lwkt_initport_replyonly(&my_autofree_rport, my_autofree_reply);

	/* Create our service thread on CPU 0. */
	lwkt_create(my_service_loop, NULL, &tdp, NULL, 0, 0, "myservice");
	my_service_port = &tdp->td_msgport;
}

/*
 * Example use case. Initialize the service and send the current thread name
 * to the service thread.
 */
static void
mod_load(void)
{
	void *arg;
	int len;

	my_service_init();
	len = strlen(curthread->td_comm);
	arg = kmalloc(len + 1, M_TEMP, M_WAITOK);
	bcopy(curthread->td_comm, arg, len + 1);
	kprintf("%s: Sending message.\en", curthread->td_comm);
	my_service_queue(arg);
}
.Ed
.Sh NOTES
All the default putport handlers (used when a message is sent) currently
implement asynchronous putports only, i.e.\& all
.Fn *_putport
handlers return
.Er EASYNC .
You can still have synchronous putport handlers (which are run in the sender's
context) but you have to implement the function yourself and then override the
default.
.Pp
Port handler functions can be overridden with custom functions if required.
You can override the default putport handler by either using the
.Fn lwkt_initport_putonly
initializer, or by manipulating the mp_putport handler pointer directly on the
.Vt lwkt_port
structure.
.Pp
There is one such case where the putport handler is overridden in
.Pa sys/net/netisr.c .
In that case, the putport handler is overridden to detect a loopback message
(when the target port belongs to the sending thread).
This special putport handler turns the sent message into a direct function call
instead of queueing it to the port.
.Pp
The
.Fn lwkt_replymsg
function works differently depending on the original message request.
If the
message was originally an asynchronous request, the reply will be queued to the
sender's reply port.
If the message was originally a synchronous request, then
this function will just write the error response on the message and wake up the
waiter without queueing the message to the reply port.
There is no need to queue in the synchronous request case because the original
sender had blocked waiting on this specific message with
.Fn lwkt_domsg .
.Pp
As is the case with putport handler, the replyport handler can also be
overridden.
You override the default replyport handler by using the
.Fn lwkt_initport_replyonly
or the
.Fn lwkt_initport_replyonly_null
port initializers, or by manipulating the mp_replyport handler pointer directly
on the
.Vt lwkt_port
structure.
.Pp
The sent message structure is reused for replies.
When a message is replied to, the error response is written on the message
which is subsequently sent to the reply port.
.Sh SEE ALSO
.Xr serializer 9 ,
.Xr sleep 9 ,
.Xr spinlock 9
.Sh HISTORY
The LWKT msgport interface first appeared in
.Dx 1.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm msgport
message passing interface implementation was written by
.An Matthew Dillon .
This manual page was written by
.An Nuno Antunes .