timeout(9)

NAME

timeout, untimeout, callout_handle_init, callout_init,

callout_init_mtx,
callout_stop, callout_drain, callout_reset, callout_pending, callout_active, callout_deactivate - execute a function af

ter a specified
length of time

SYNOPSIS

#include <sys/types.h>
#include <sys/systm.h>
typedef void timeout_t (void *);
struct callout_handle
timeout(timeout_t *func, void *arg, int ticks);
void
callout_handle_init(struct callout_handle *handle);
struct  callout_handle  handle  =  CALLOUT_HANDLE_INITIALIZER(&handle)
void
untimeout(timeout_t *func, void *arg, struct  callout_handle
handle);
void
callout_init(struct callout *c, int mpsafe);
void
callout_init_mtx(struct  callout  *c,  struct  mtx *mtx, int
flags);
int
callout_stop(struct callout *c);
int
callout_drain(struct callout *c);
int
callout_reset(struct callout *c, int ticks, timeout_t *func,
void *arg);
int
callout_pending(struct callout *c);
int
callout_active(struct callout *c);
callout_deactivate(struct callout *c);

DESCRIPTION

The function timeout() schedules a call to the function giv

en by the
argument func to take place after ticks/hz seconds. Non

positive values
of ticks are silently converted to the value `1'. func

should be a
pointer to a function that takes a void * argument. Upon

invocation,
func will receive arg as its only argument. The return val

ue from
timeout() is a struct callout_handle which can be used in

conjunction
with the untimeout() function to request that a scheduled

timeout be canceled. The timeout() call is the old style and new code

should use the
callout_*() functions.

The function callout_handle_init() can be used to initialize

a handle to
a state which will cause any calls to untimeout() with that

handle to
return with no side effects.

Assigning a callout handle the value of

CALLOUT_HANDLE_INITIALIZER

forms the same function as callout_handle_init() and is pro

vided for use
on statically declared or global callout handles.

The function untimeout() cancels the timeout associated with

handle using
the func and arg arguments to validate the handle. If the

handle does
not correspond to a timeout with the function func taking

the argument
arg no action is taken. handle must be initialized by a

previous call to
timeout(), callout_handle_init(), or assigned the value of CALLOUT_HANDLE_INITIALIZER(&handle) before being passed to

untimeout().
The behavior of calling untimeout() with an uninitialized

handle is undefined. The untimeout() call is the old style and new code

should use the
callout_*() functions.

As handles are recycled by the system, it is possible (al

though unlikely)
that a handle from one invocation of timeout() may match the

handle of
another invocation of timeout() if both calls used the same

function
pointer and argument, and the first timeout is expired or

canceled before
the second call. The timeout facility offers O(1) running

time for
timeout() and untimeout(). Timeouts are executed from

softclock() with
the Giant lock held. Thus they are protected from re-en

trancy.

The functions callout_init(), callout_init_mtx(),

callout_stop(),
callout_drain() and callout_reset() are low-level routines

for clients
who wish to allocate their own callout structures.

The function callout_init() initializes a callout so it can

be passed to
callout_stop(), callout_drain() or callout_reset() without

any side
effects. If the mpsafe argument is zero, the callout struc

ture is not
considered to be ``multi-processor safe''; that is, the Gi

ant lock will
be acquired before calling the callout function, and re

leased when the
callout function returns.

The callout_init_mtx() function may be used as an alterna

tive to
callout_init(). The parameter mtx specifies a mutex that is

to be
acquired by the callout subsystem before calling the callout

function,
and released when the callout function returns. The follow

ing flags may
be specified:

CALLOUT_RETURNUNLOCKED The callout function will re

lease mtx itself,

so the callout subsystem should

not attempt
to unlock it after the callout

function
returns.

The function callout_stop() cancels a callout if it is cur

rently pending.
If the callout is pending, then callout_stop() will return a

non-zero
value. If the callout is not set, has already been serviced

or is currently being serviced, then zero will be returned. If the

callout has an
associated mutex, then that mutex must be held when this

function is
called.

The function callout_drain() is identical to callout_stop()

except that
it will wait for the callout to be completed if it is al

ready in
progress. This function MUST NOT be called while holding

any locks on
which the callout might block, or deadlock will result.

Note that if the
callout subsystem has already begun processing this callout,

then the
callout function may be invoked during the execution of

callout_drain().
However, the callout subsystem does guarantee that the call

out will be
fully stopped before callout_drain() returns.

The function callout_reset() first performs the equivalent

of
callout_stop() to disestablish the callout, and then estab

lishes a new
callout in the same manner as timeout(). If there was al

ready a pending
callout and it was rescheduled, then callout_reset() will

return a nonzero value. If the callout has an associated mutex, then

that mutex must
be held when this function is called.

The macros callout_pending(), callout_active() and

callout_deactivate()
provide access to the current state of the callout. Careful

use of these
macros can avoid many of the race conditions that are inher

ent in asynchronous timer facilities; see Avoiding Race Conditions be

low for further
details. The callout_pending() macro checks whether a call

out is
pending; a callout is considered pending when a timeout has

been set but
the time has not yet arrived. Note that once the timeout

time arrives
and the callout subsystem starts to process this callout,
callout_pending() will return FALSE even though the callout

function may
not have finished (or even begun) executing. The

callout_active() macro
checks whether a callout is marked as active, and the
callout_deactivate() macro clears the callout's active flag.

The callout
subsystem marks a callout as active when a timeout is set

and it clears
the active flag in callout_stop() and callout_drain(), but

it does not
clear it when a callout expires normally via the execution

of the callout
function.

Avoiding Race Conditions

The callout subsystem invokes callout functions from its own

timer context. Without some kind of synchronization it is possible

that a callout
function will be invoked concurrently with an attempt to

stop or reset
the callout by another thread. In particular, since callout

functions
typically acquire a mutex as their first action, the callout

function may
have already been invoked, but be blocked waiting for that

mutex at the
time that another thread tries to reset or stop the callout.

The callout subsystem provides a number of mechanisms to ad

dress these
synchronization concerns:

1. If the callout has an associated mutex that was

specifiedusing the callout_init_mtx() function (or implic

itly specified
as the Giant mutex using callout_init() with

mpsafe set to
FALSE), then this mutex is used to avoid the race

conditions.
The associated mutex must be acquired by the

caller before
calling callout_stop() or callout_reset() and it

is guaranteed
that the callout will be correctly stopped or re

set as
expected. Note that it is still necessary to use
callout_drain() before destroying the callout or

its associated mutex.

2. The return value from callout_stop() and

callout_reset() indi
cates whether or not the callout was removed. If

it is known
that the callout was set and the callout function

has not yet
executed, then a return value of FALSE indicates

that the
callout function is about to be called. For ex

ample:

if (sc->sc_flags & SCFLG_CALLOUT_RUNNING) {
if (callout_stop(&sc->sc_callout))

{
sc->sc_flags &=

~SCFLG_CALLOUT_RUNNING;
/* successfully stopped */

} else {
/*
* callout has expired and

callout
* function is about to be

executed
*/

}

}

3. The callout_pending(), callout_active() and callout_deactivate() macros can be used together

to work
around the race conditions. When a callout's

timeout is set,
the callout subsystem marks the callout as both

active and
pending. When the timeout time arrives, the

callout subsystem
begins processing the callout by first clearing

the pending
flag. It then invokes the callout function with

out changing
the active flag, and does not clear the active

flag even after
the callout function returns. The mechanism de

scribed here
requires the callout function itself to clear the

active flag
using the callout_deactivate() macro. The

callout_stop() and
callout_drain() functions always clear both the

active and
pending flags before returning.

The callout function should first check the

pending flag and
return without action if callout_pending() re

turns TRUE. This
indicates that the callout was rescheduled using
callout_reset() just before the callout function

was invoked.
If callout_active() returns FALSE then the call

out function
should also return without action. This indi

cates that the
callout has been stopped. Finally, the callout

function
should call callout_deactivate() to clear the

active flag.
For example:

mtx_lock(&sc->sc_mtx);
if (callout_pending(&sc->sc_callout)) {
/* callout was reset */
mtx_unlock(&sc->sc_mtx);
return;

}
if (!callout_active(&sc->sc_callout)) {
/* callout was stopped */
mtx_unlock(&sc->sc_mtx);
return;

}
callout_deactivate(&sc->sc_callout);
/* rest of callout function */

Together with appropriate synchronization, such

as the mutex
used above, this approach permits the

callout_stop() and
callout_reset() functions to be used at any time

without
races. For example:

mtx_lock(&sc->sc_mtx);
callout_stop(&sc->sc_callout);
/* The callout is effectively stopped now.

*/

If the callout is still pending then these func

tions operate
normally, but if processing of the callout has

already begun
then the tests in the callout function cause it

to return
without further action. Synchronization between

the callout
function and other code ensures that stopping or

resetting the
callout will never be attempted while the callout

function is
past the callout_deactivate() call.

The above technique additionally ensures that the

active flag
always reflects whether the callout is effective

ly enabled or
disabled. If callout_active() returns false,

then the callout
is effectively disabled, since even if the call

out subsystem
is actually just about to invoke the callout

function, the
callout function will return without action.

There is one final race condition that must be considered

when a callout
is being stopped for the last time. In this case it may not

be safe to
let the callout function itself detect that the callout was

stopped,
since it may need to access data objects that have already

been destroyed
or recycled. To ensure that the callout is completely fin

ished, a call
to callout_drain() should be used.

RETURN VALUES

The timeout() function returns a struct callout_handle that

can be passed
to untimeout(). The callout_stop() and callout_drain()

functions return
non-zero if the callout was still pending when it was called

or zero otherwise.

HISTORY

The current timeout and untimeout routines are based on the

work of Adam
M. Costello and George Varghese, published in a technical

report entitled
Redesigning the BSD Callout and Timer Facilities and modi

fied slightly
for inclusion in FreeBSD by Justin T. Gibbs. The original

work on the
data structures used in this implementation was published by

G. Varghese
and A. Lauck in the paper Hashed and Hierarchical Timing

Wheels: Data
Structures for the Efficient Implementation of a Timer

Facility in the
Proceedings of the 11th ACM Annual Symposium on Operating

Systems
Principles. The current implementation replaces the long

standing BSD
linked list callout mechanism which offered O(n) insertion

and removal
running time but did not generate or require handles for un

timeout operations.

BSD September 8, 2005

BSD