Provided by: freebsd-manpages_12.2-2_all 
NAME
callout_active, callout_deactivate, callout_async_drain, callout_drain, callout_handle_init,
callout_init, callout_init_mtx, callout_init_rm, callout_init_rw, callout_pending, callout_reset,
callout_reset_curcpu, callout_reset_on, callout_reset_sbt, callout_reset_sbt_curcpu,
callout_reset_sbt_on, callout_schedule, callout_schedule_curcpu, callout_schedule_on,
callout_schedule_sbt, callout_schedule_sbt_curcpu, callout_schedule_sbt_on, callout_stop, callout_when,
timeout, untimeout — execute a function after a specified length of time
SYNOPSIS
#include <sys/types.h>
#include <sys/callout.h>
#include <sys/systm.h>
typedef void callout_func_t (void *);
typedef void timeout_t (void *);
int
callout_active(struct callout *c);
void
callout_deactivate(struct callout *c);
int
callout_async_drain(struct callout *c, callout_func_t *drain);
int
callout_drain(struct callout *c);
void
callout_handle_init(struct callout_handle *handle);
struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle);
void
callout_init(struct callout *c, int mpsafe);
void
callout_init_mtx(struct callout *c, struct mtx *mtx, int flags);
void
callout_init_rm(struct callout *c, struct rmlock *rm, int flags);
void
callout_init_rw(struct callout *c, struct rwlock *rw, int flags);
int
callout_pending(struct callout *c);
int
callout_reset(struct callout *c, int ticks, callout_func_t *func, void *arg);
int
callout_reset_curcpu(struct callout *c, int ticks, callout_func_t *func, void *arg);
int
callout_reset_on(struct callout *c, int ticks, callout_func_t *func, void *arg, int cpu);
int
callout_reset_sbt(struct callout *c, sbintime_t sbt, sbintime_t pr, callout_func_t *func, void *arg,
int flags);
int
callout_reset_sbt_curcpu(struct callout *c, sbintime_t sbt, sbintime_t pr, callout_func_t *func,
void *arg, int flags);
int
callout_reset_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t pr, callout_func_t *func, void *arg,
int cpu, int flags);
int
callout_schedule(struct callout *c, int ticks);
int
callout_schedule_curcpu(struct callout *c, int ticks);
int
callout_schedule_on(struct callout *c, int ticks, int cpu);
int
callout_schedule_sbt(struct callout *c, sbintime_t sbt, sbintime_t pr, int flags);
int
callout_schedule_sbt_curcpu(struct callout *c, sbintime_t sbt, sbintime_t pr, int flags);
int
callout_schedule_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t pr, int cpu, int flags);
int
callout_stop(struct callout *c);
sbintime_t
callout_when(sbintime_t sbt, sbintime_t precision, int flags, sbintime_t *sbt_res,
sbintime_t *precision_res);
struct callout_handle
timeout(timeout_t *func, void *arg, int ticks);
void
untimeout(timeout_t *func, void *arg, struct callout_handle handle);
DESCRIPTION
The callout API is used to schedule a call to an arbitrary function at a specific time in the future.
Consumers of this API are required to allocate a callout structure (struct callout) for each pending
function invocation. This structure stores state about the pending function invocation including the
function to be called and the time at which the function should be invoked. Pending function calls can
be cancelled or rescheduled to a different time. In addition, a callout structure may be reused to
schedule a new function call after a scheduled call is completed.
Callouts only provide a single-shot mode. If a consumer requires a periodic timer, it must explicitly
reschedule each function call. This is normally done by rescheduling the subsequent call within the
called function.
Callout functions must not sleep. They may not acquire sleepable locks, wait on condition variables,
perform blocking allocation requests, or invoke any other action that might sleep.
Each callout structure must be initialized by callout_init(), callout_init_mtx(), callout_init_rm(), or
callout_init_rw() before it is passed to any of the other callout functions. The callout_init() function
initializes a callout structure in c that is not associated with a specific lock. If the mpsafe argument
is zero, the callout structure is not considered to be “multi-processor safe”; and the Giant lock will be
acquired before calling the callout function and released when the callout function returns.
The callout_init_mtx(), callout_init_rm(), and callout_init_rw() functions initialize a callout structure
in c that is associated with a specific lock. The lock is specified by the mtx, rm, or rw parameter.
The associated lock must be held while stopping or rescheduling the callout. The callout subsystem
acquires the associated lock before calling the callout function and releases it after the function
returns. If the callout was cancelled while the callout subsystem waited for the associated lock, the
callout function is not called, and the associated lock is released. This ensures that stopping or
rescheduling the callout will abort any previously scheduled invocation.
Only regular mutexes may be used with callout_init_mtx(); spin mutexes are not supported. A sleepable
read-mostly lock (one initialized with the RM_SLEEPABLE flag) may not be used with callout_init_rm().
Similarly, other sleepable lock types such as sx(9) and lockmgr(9) cannot be used with callouts because
sleeping is not permitted in the callout subsystem.
These flags may be specified for callout_init_mtx(), callout_init_rm(), or callout_init_rw():
CALLOUT_RETURNUNLOCKED The callout function will release the associated lock itself, so the callout
subsystem should not attempt to unlock it after the callout function returns.
CALLOUT_SHAREDLOCK The lock is only acquired in read mode when running the callout handler. This
flag is ignored by callout_init_mtx().
The function callout_stop() cancels a callout c if it is currently pending. If the callout is pending
and successfully stopped, then callout_stop() returns a value of one. If the callout is not set, or has
already been serviced, then negative one is returned. If the callout is currently being serviced and
cannot be stopped, then zero will be returned. If the callout is currently being serviced and cannot be
stopped, and at the same time a next invocation of the same callout is also scheduled, then
callout_stop() unschedules the next run and returns zero. If the callout has an associated lock, then
that lock must be held when this function is called.
The function callout_async_drain() is identical to callout_stop() with one difference. When
callout_async_drain() returns zero it will arrange for the function drain to be called using the same
argument given to the callout_reset() function. callout_async_drain() If the callout has an associated
lock, then that lock must be held when this function is called. Note that when stopping multiple
callouts that use the same lock it is possible to get multiple return's of zero and multiple calls to the
drain function, depending upon which CPU's the callouts are running. The drain function itself is called
from the context of the completing callout i.e. softclock or hardclock, just like a callout itself.
The function callout_drain() is identical to callout_stop() except that it will wait for the callout c to
complete if it is already 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 before callout_drain() returns.
However, the callout subsystem does guarantee that the callout will be fully stopped before
callout_drain() returns.
The callout_reset() and callout_schedule() function families schedule a future function invocation for
callout c. If c already has a pending callout, it is cancelled before the new invocation is scheduled.
These functions return a value of one if a pending callout was cancelled and zero if there was no pending
callout. If the callout has an associated lock, then that lock must be held when any of these functions
are called.
The time at which the callout function will be invoked is determined by either the ticks argument or the
sbt, pr, and flags arguments. When ticks is used, the callout is scheduled to execute after ticks/hz
seconds. Non-positive values of ticks are silently converted to the value ‘1’.
The sbt, pr, and flags arguments provide more control over the scheduled time including support for
higher resolution times, specifying the precision of the scheduled time, and setting an absolute deadline
instead of a relative timeout. The callout is scheduled to execute in a time window which begins at the
time specified in sbt and extends for the amount of time specified in pr. If sbt specifies a time in the
past, the window is adjusted to start at the current time. A non-zero value for pr allows the callout
subsystem to coalesce callouts scheduled close to each other into fewer timer interrupts, reducing
processing overhead and power consumption. These flags may be specified to adjust the interpretation of
sbt and pr:
C_ABSOLUTE Handle the sbt argument as an absolute time since boot. By default, sbt is treated as a
relative amount of time, similar to ticks.
C_DIRECT_EXEC Run the handler directly from hardware interrupt context instead of from the softclock
thread. This reduces latency and overhead, but puts more constraints on the callout
function. Callout functions run in this context may use only spin mutexes for locking and
should be as small as possible because they run with absolute priority.
C_PREL() Specifies relative event time precision as binary logarithm of time interval divided by
acceptable time deviation: 1 -- 1/2, 2 -- 1/4, etc. Note that the larger of pr or this
value is used as the length of the time window. Smaller values (which result in larger
time intervals) allow the callout subsystem to aggregate more events in one timer
interrupt.
C_PRECALC The sbt argument specifies the absolute time at which the callout should be run, and the
pr argument specifies the requested precision, which will not be adjusted during the
scheduling process. The sbt and pr values should be calculated by an earlier call to
callout_when() which uses the user-supplied sbt, pr, and flags values.
C_HARDCLOCK Align the timeouts to hardclock() calls if possible.
The callout_reset() functions accept a func argument which identifies the function to be called when the
time expires. It must be a pointer to a function that takes a single void * argument. Upon invocation,
func will receive arg as its only argument. The callout_schedule() functions reuse the func and arg
arguments from the previous callout. Note that one of the callout_reset() functions must always be
called to initialize func and arg before one of the callout_schedule() functions can be used.
The callout subsystem provides a softclock thread for each CPU in the system. Callouts are assigned to a
single CPU and are executed by the softclock thread for that CPU. Initially, callouts are assigned to
CPU 0. The callout_reset_on(), callout_reset_sbt_on(), callout_schedule_on() and
callout_schedule_sbt_on() functions assign the callout to CPU cpu. The callout_reset_curcpu(),
callout_reset_sbt_curpu(), callout_schedule_curcpu() and callout_schedule_sbt_curcpu() functions assign
the callout to the current CPU. The callout_reset(), callout_reset_sbt(), callout_schedule() and
callout_schedule_sbt() functions schedule the callout to execute in the softclock thread of the CPU to
which it is currently assigned.
Softclock threads are not pinned to their respective CPUs by default. The softclock thread for CPU 0 can
be pinned to CPU 0 by setting the kern.pin_default_swi loader tunable to a non-zero value. Softclock
threads for CPUs other than zero can be pinned to their respective CPUs by setting the kern.pin_pcpu_swi
loader tunable to a non-zero value.
The macros callout_pending(), callout_active() and callout_deactivate() provide access to the current
state of the callout. The callout_pending() macro checks whether a callout 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.
The callout_when() function may be used to pre-calculate the absolute time at which the timeout should be
run and the precision of the scheduled run time according to the required time sbt, precision precision,
and additional adjustments requested by the flags argument. Flags accepted by the callout_when()
function are the same as flags for the callout_reset() function. The resulting time is assigned to the
variable pointed to by the sbt_res argument, and the resulting precision is assigned to *precision_res.
When passing the results to callout_reset, add the C_PRECALC flag to flags, to avoid incorrect re-
adjustment. The function is intended for situations where precise time of the callout run should be
known in advance, since trying to read this time from the callout structure itself after a
callout_reset() call is racy.
Avoiding Race Conditions
The callout subsystem invokes callout functions from its own thread 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
lock as their first action, the callout function may have already been invoked, but is blocked waiting
for that lock at the time that another thread tries to reset or stop the callout.
There are three main techniques for addressing these synchronization concerns. The first approach is
preferred as it is the simplest:
1. Callouts can be associated with a specific lock when they are initialized by
callout_init_mtx(), callout_init_rm(), or callout_init_rw(). When a callout is associated
with a lock, the callout subsystem acquires the lock before the callout function is invoked.
This allows the callout subsystem to transparently handle races between callout cancellation,
scheduling, and execution. Note that the associated lock must be acquired before calling
callout_stop() or one of the callout_reset() or callout_schedule() functions to provide this
safety.
A callout initialized via callout_init() with mpsafe set to zero is implicitly associated with
the Giant mutex. If Giant is held when cancelling or rescheduling the callout, then its use
will prevent races with the callout function.
2. The return value from callout_stop() (or the callout_reset() and callout_schedule() function
families) indicates 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 example:
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 without changing the active flag, and does not clear the active flag even
after the callout function returns. The mechanism described 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() returns 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 callout function should also return without action. This indicates 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 functions 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 effectively enabled or disabled. If callout_active() returns false, then the
callout is effectively disabled, since even if the callout 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 finished, a call to callout_drain() should be used. In particular,
a callout should always be drained prior to destroying its associated lock or releasing the storage for
the callout structure.
LEGACY API
The functions below are a legacy API that will be removed in a future release. New code should not use
these routines.
The function timeout() schedules a call to the function given 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 value 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 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() performs the same function as
callout_handle_init() and is provided 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.
As handles are recycled by the system, it is possible (although 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-entrancy.
RETURN VALUES
The callout_active() macro returns the state of a callout's active flag.
The callout_pending() macro returns the state of a callout's pending flag.
The callout_reset() and callout_schedule() function families return a value of one if the callout was
pending before the new function invocation was scheduled.
The callout_stop() and callout_drain() functions return a value of one if the callout was still pending
when it was called, a zero if the callout could not be stopped and a negative one is it was either not
running or has already completed. The timeout() function returns a struct callout_handle that can be
passed to untimeout().
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 modified
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 untimeout operations.
Debian December 10, 2019 TIMEOUT(9)