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     timeout, untimeout, callout_handle_init, callout_init, callout_init_mtx,
     callout_init_rw, callout_stop, callout_drain, callout_reset,
     callout_schedule, callout_pending, callout_active, callout_deactivate —
     execute a function after a specified length of time


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

     typedef void timeout_t (void *);

     struct callout_handle
     timeout(timeout_t *func, void *arg, int ticks);

     callout_handle_init(struct callout_handle *handle);

     struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle)

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

     callout_init(struct callout *c, int mpsafe);

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

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

     callout_stop(struct callout *c);

     callout_drain(struct callout *c);

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

     callout_schedule(struct callout *c, int ticks);

     callout_pending(struct callout *c);

     callout_active(struct callout *c);

     callout_deactivate(struct callout *c);


     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 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()
     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.  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 (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.

     The functions callout_init(), callout_init_mtx(), callout_init_rw(),
     callout_stop(), callout_drain(), callout_reset() and callout_schedule()
     are low-level routines for clients who wish to allocate their own callout

     The function callout_init() initializes a callout so it can be passed to
     callout_stop(), callout_drain(), callout_reset() or callout_schedule()
     without any side effects.  If the mpsafe argument is zero, the callout
     structure is not considered to be “multi-processor safe”; that is, the
     Giant lock will be acquired before calling the callout function, and
     released when the callout function returns.

     The callout_init_mtx() function may be used as an alternative 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 following flags may
     be specified:

     CALLOUT_RETURNUNLOCKED  The callout function will release mtx itself, so
                             the callout subsystem should not attempt to
                             unlock it after the callout function returns.

     The callout_init_rw() function serves the need of using rwlocks in
     conjunction with callouts.  The function does basically the same as
     callout_init_mtx() with the possibility of specifying an extra rw
     argument.  The usable lock classes are currently limited to mutexes and
     rwlocks, because callout handlers run in softclock swi, so they cannot
     sleep nor acquire sleepable locks like sx or lockmgr.  The following
     flags may be specified:

     CALLOUT_SHAREDLOCK  The lock is only acquired in read mode when running
                         the callout handler.  It has no effects when used in
                         conjunction with mtx.

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

     The function callout_drain() is identical to callout_stop() except that
     it will wait for the callout to be completed 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 during the execution of callout_drain().
     However, the callout subsystem does guarantee that the callout 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 establishes a new
     callout in the same manner as timeout().  If there was already a pending
     callout and it was rescheduled, then callout_reset() will return a non-
     zero value.  If the callout has an associated mutex, then that mutex must
     be held when this function is called.  The function callout_schedule()
     (re)schedules an existing callout for a new period of time; it is
     equivalent to calling callout_reset() with the func and arg parameters
     extracted from the callout structure (though possibly with lower

     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 inherent in
     asynchronous timer facilities; see Avoiding Race Conditions below for
     further details.  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

   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 address these
     synchronization concerns:

           1.   If the callout has an associated mutex that was specified
                using the callout_init_mtx() function (or implicitly 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 reset 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()
                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:

                      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 callout_stop() 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 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.


     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


     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

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