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TIMEOUT(9) Kernel Developer's Manual TIMEOUT(9)


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, untimeoutexecute a function after a specified length of time


#include <sys/types.h>
#include <sys/callout.h>
#include <sys/systm.h>

typedef void callout_func_t (void *);
typedef void timeout_t (void *);

callout_active(struct callout *c);

callout_deactivate(struct callout *c);

callout_async_drain(struct callout *c, callout_func_t *drain);

callout_drain(struct callout *c);

callout_handle_init(struct callout_handle *handle);

struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle);

callout_init(struct callout *c, int mpsafe);

callout_init_mtx(struct callout *c, struct mtx *mtx, int flags);

callout_init_rm(struct callout *c, struct rmlock *rm, int flags);

callout_init_rw(struct callout *c, struct rwlock *rw, int flags);

callout_pending(struct callout *c);

callout_reset(struct callout *c, int ticks, callout_func_t *func, void *arg);

callout_reset_curcpu(struct callout *c, int ticks, callout_func_t *func, void *arg);

callout_reset_on(struct callout *c, int ticks, callout_func_t *func, void *arg, int cpu);

callout_reset_sbt(struct callout *c, sbintime_t sbt, sbintime_t pr, callout_func_t *func, void *arg, int flags);

callout_reset_sbt_curcpu(struct callout *c, sbintime_t sbt, sbintime_t pr, callout_func_t *func, void *arg, int flags);

callout_reset_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t pr, callout_func_t *func, void *arg, int cpu, int flags);

callout_schedule(struct callout *c, int ticks);

callout_schedule_curcpu(struct callout *c, int ticks);

callout_schedule_on(struct callout *c, int ticks, int cpu);

callout_schedule_sbt(struct callout *c, sbintime_t sbt, sbintime_t pr, int flags);

callout_schedule_sbt_curcpu(struct callout *c, sbintime_t sbt, sbintime_t pr, int flags);

callout_schedule_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t pr, int cpu, int flags);

callout_stop(struct callout *c);

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);

untimeout(timeout_t *func, void *arg, struct callout_handle handle);


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_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 (), (), and () 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 (); spin mutexes are not supported. A sleepable read-mostly lock (one initialized with the RM_SLEEPABLE flag) may not be used with (). 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 (), (), or ():

The callout function will release the associated lock itself, so the callout subsystem should not attempt to unlock it after the callout function returns.
The lock is only acquired in read mode when running the callout handler. This flag is ignored by callout_init_mtx().

The function () 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 () 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 () 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 () and () 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:

Handle the sbt argument as an absolute time since boot. By default, sbt is treated as a relative amount of time, similar to ticks.
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.
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.
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.
Align the timeouts to () calls if possible.

The () 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 () 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 (), (), () and () functions assign the callout to CPU cpu. The (), (), () and () functions assign the callout to the current CPU. The callout_reset(), (), callout_schedule() and () 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_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 clear it when a callout expires normally via the execution of the callout function.

The () 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 (), (), or (). 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 () 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 () 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:

    if (callout_pending(&sc->sc_callout)) {
    	/* callout was reset */
    if (!callout_active(&sc->sc_callout)) {
    	/* callout was stopped */
    /* rest of callout function */

    Together with appropriate synchronization, such as the mutex used above, this approach permits the () and callout_reset() functions to be used at any time without races. For example:

    /* 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 () call.

    The above technique additionally ensures that the active flag always reflects whether the callout is effectively enabled or disabled. If () 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 () 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.


The functions below are a legacy API that will be removed in a future release. New code should not use these routines.

The function () 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 () 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 () performs the same function as callout_handle_init() and is provided for use on statically declared or global callout handles.

The function () 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 () 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 () with the Giant lock held. Thus they are protected from re-entrancy.


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().


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.

December 10, 2019 Debian