Provided by: freebsd-manpages_12.2-1_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.