Rewrite timeout(9) to be callout(9)-centric instead. Move the description

of timeout(9) to the end and mark it prominently as deprecated.  Document
somewhat how times are specified for the 'sbt' variants.  Better explain
how using callout_init_*() to associate a lock with a callout resolves
common races.

Differential Revision:	https://reviews.freebsd.org/D847
Reviewed by:	wblock, bjk
MFC after:	1 week

share/man/man9/timeout.9

@@ -33,27 +33,27 @@
 .Dt TIMEOUT 9
 .Os
 .Sh NAME
-.Nm timeout ,
-.Nm untimeout ,
+.Nm callout_active ,
+.Nm callout_deactivate ,
+.Nm callout_drain ,
 .Nm callout_handle_init ,
 .Nm callout_init ,
 .Nm callout_init_mtx ,
 .Nm callout_init_rm ,
 .Nm callout_init_rw ,
-.Nm callout_stop ,
-.Nm callout_drain ,
-.Nm callout_reset ,
-.Nm callout_reset_on ,
-.Nm callout_reset_curcpu ,
-.Nm callout_reset_sbt ,
-.Nm callout_reset_sbt_on ,
-.Nm callout_reset_sbt_curcpu ,
-.Nm callout_schedule ,
-.Nm callout_schedule_on ,
-.Nm callout_schedule_curcpu ,
 .Nm callout_pending ,
-.Nm callout_active ,
-.Nm callout_deactivate
+.Nm callout_reset ,
+.Nm callout_reset_curcpu ,
+.Nm callout_reset_on ,
+.Nm callout_reset_sbt ,
+.Nm callout_reset_sbt_curcpu ,
+.Nm callout_reset_sbt_on ,
+.Nm callout_schedule ,
+.Nm callout_schedule_curcpu ,
+.Nm callout_schedule_on ,
+.Nm callout_stop ,
+.Nm timeout ,
+.Nm untimeout
 .Nd execute a function after a specified length of time
 .Sh SYNOPSIS
 .In sys/types.h
@@ -61,16 +61,18 @@
 .Bd -literal
 typedef void timeout_t (void *);
 .Ed
-.Ft struct callout_handle
-.Fn timeout "timeout_t *func" "void *arg" "int ticks"
+.Ft int
+.Fn callout_active "struct callout *c"
+.Ft void
+.Fn callout_deactivate "struct callout *c"
+.Ft int
+.Fn callout_drain "struct callout *c"
 .Ft void
 .Fn callout_handle_init "struct callout_handle *handle"
 .Bd -literal
 struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle);
 .Ed
 .Ft void
-.Fn untimeout "timeout_t *func" "void *arg" "struct callout_handle handle"
-.Ft void
 .Fn callout_init "struct callout *c" "int mpsafe"
 .Ft void
 .Fn callout_init_mtx "struct callout *c" "struct mtx *mtx" "int flags"
@@ -79,46 +81,279 @@ struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle);
 .Ft void
 .Fn callout_init_rw "struct callout *c" "struct rwlock *rw" "int flags"
 .Ft int
-.Fn callout_stop "struct callout *c"
-.Ft int
-.Fn callout_drain "struct callout *c"
+.Fn callout_pending "struct callout *c"
 .Ft int
 .Fn callout_reset "struct callout *c" "int ticks" "timeout_t *func" "void *arg"
 .Ft int
-.Fn callout_reset_on "struct callout *c" "int ticks" "timeout_t *func" \
-"void *arg" "int cpu"
-.Ft int
-.Fn callout_reset_sbt_on "struct callout *c" "sbintime_t sbt" \
-"sbintime_t pr" "timeout_t *func" "void *arg" "int cpu" "int flags"
-.Ft int
 .Fn callout_reset_curcpu "struct callout *c" "int ticks" "timeout_t *func" \
 "void *arg"
 .Ft int
-.Fn callout_schedule "struct callout *c" "int ticks"
+.Fn callout_reset_on "struct callout *c" "int ticks" "timeout_t *func" \
+"void *arg" "int cpu"
 .Ft int
-.Fn callout_schedule_on "struct callout *c" "int ticks" "int cpu"
+.Fn callout_reset_sbt "struct callout *c" "sbintime_t sbt" \
+"sbintime_t pr" "timeout_t *func" "void *arg" "int flags"
+.Ft int
+.Fn callout_reset_sbt_curcpu "struct callout *c" "sbintime_t sbt" \
+"sbintime_t pr" "timeout_t *func" "void *arg" "int flags"
+.Ft int
+.Fn callout_reset_sbt_on "struct callout *c" "sbintime_t sbt" \
+"sbintime_t pr" "timeout_t *func" "void *arg" "int cpu" "int flags"
+.Ft int
+.Fn callout_schedule "struct callout *c" "int ticks"
 .Ft int
 .Fn callout_schedule_curcpu "struct callout *c" "int ticks"
 .Ft int
-.Fn callout_pending "struct callout *c"
+.Fn callout_schedule_on "struct callout *c" "int ticks" "int cpu"
 .Ft int
-.Fn callout_active "struct callout *c"
+.Fn callout_stop "struct callout *c"
+.Ft struct callout_handle
+.Fn timeout "timeout_t *func" "void *arg" "int ticks"
 .Ft void
-.Fn callout_deactivate "struct callout *c"
+.Fn untimeout "timeout_t *func" "void *arg" "struct callout_handle handle"
 .Sh DESCRIPTION
+The
+.Nm 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
+.Pq 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.
+.Pp
+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.
+.Pp
+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.
+.Pp
+Each callout structure must be initialized by
+.Fn callout_init ,
+.Fn callout_init_mtx ,
+.Fn callout_init_rm ,
+or
+.Fn callout_init_rw
+before it is passed to any of the other callout functions.
+The
+.Fn callout_init
+function initializes a callout structure in
+.Fa c
+that is not associated with a specific lock.
+If the
+.Fa mpsafe
+argument is zero,
+the callout structure is not considered to be
+.Dq multi-processor safe ;
+and the Giant lock will be acquired before calling the callout function
+and released when the callout function returns.
+.Pp
+The
+.Fn callout_init_mtx ,
+.Fn callout_init_rm ,
+and
+.Fn callout_init_rw
+functions initialize a callout structure in
+.Fa c
+that is associated with a specific lock.
+The lock is specified by the
+.Fa mtx ,
+.Fa rm ,
+or
+.Fa 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.
+.Pp
+Only regular mutexes may be used with
+.Fn callout_init_mtx ;
+spin mutexes are not supported.
+A sleepable read-mostly lock
+.Po
+one initialized with the
+.Dv RM_SLEEPABLE
+flag
+.Pc
+may not be used with
+.Fn callout_init_rm .
+Similarly, other sleepable lock types such as
+.Xr sx 9
+and
+.Xr lockmgr 9
+cannot be used with callouts because sleeping is not permitted in
+the callout subsystem.
+.Pp
+These
+.Fa flags
+may be specified for
+.Fn callout_init_mtx ,
+.Fn callout_init_rm ,
+or
+.Fn callout_init_rw :
+.Bl -tag -width ".Dv CALLOUT_RETURNUNLOCKED"
+.It Dv 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.
+.It Dv CALLOUT_SHAREDLOCK
+The lock is only acquired in read mode when running the callout handler.
+This flag is ignored by
+.Fn callout_init_mtx .
+.El
+.Pp
 The function
-.Fn timeout
-schedules a call to the function given by the argument
-.Fa func
-to take place after
+.Fn callout_stop
+cancels a callout
+.Fa c
+if it is currently pending.
+If the callout is pending, then
+.Fn callout_stop
+returns 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 lock,
+then that lock must be held when this function is called.
+.Pp
+The function
+.Fn callout_drain
+is identical to
+.Fn callout_stop
+except that it will wait for the callout
+.Fa 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
+.Fn callout_drain
+returns.
+However, the callout subsystem does guarantee that the callout will be
+fully stopped before
+.Fn callout_drain
+returns.
+.Pp
+The
+.Fn callout_reset
+and
+.Fn callout_schedule
+function families schedule a future function invocation for callout
+.Fa c .
+If
+.Fa c
+already has a pending callout,
+it is cancelled before the new invocation is scheduled.
+These functions return a non-zero value 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.
+.Pp
+The time at which the callout function will be invoked is determined by
+either the
+.Fa ticks
+argument or the
+.Fa sbt ,
+.Fa pr ,
+and
+.Fa flags
+arguments.
+When
+.Fa ticks
+is used,
+the callout is scheduled to execute after
 .Fa ticks Ns No /hz
 seconds.
 Non-positive values of
 .Fa ticks
 are silently converted to the value
 .Sq 1 .
+.Pp
+The
+.Fa sbt ,
+.Fa pr ,
+and
+.Fa 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
+.Fa sbt
+and extends for the amount of time specified in
+.Fa pr .
+If
+.Fa sbt
+specifies a time in the past,
+the window is adjusted to start at the current time.
+A non-zero value for
+.Fa pr
+allows the callout subsystem to coalesce callouts scheduled close to each
+other into fewer timer interrupts,
+reducing processing overhead and power consumption.
+These
+.Fa flags
+may be specified to adjust the interpretation of
+.Fa sbt
+and
+.Fa pr :
+.Bl -tag -width ".Dv C_DIRECT_EXEC"
+.It Dv C_ABSOLUTE
+Handle the
+.Fa sbt
+argument as an absolute time since boot.
+By default,
+.Fa sbt
+is treated as a relative amount of time,
+similar to
+.Fa ticks .
+.It Dv 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.
+.It Fn 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
+.Fa pr
+or this value is used as the length of the time window.
+Smaller values
+.Pq which result in larger time intervals
+allow the callout subsystem to aggregate more events in one timer interrupt.
+.It Dv C_HARDCLOCK
+Align the timeouts to
+.Fn hardclock
+calls if possible.
+.El
+.Pp
+The
+.Fn callout_reset
+functions accept a
 .Fa func
-should be a pointer to a function that takes a
+argument which identifies the function to be called when the time expires.
+It must be a pointer to a function that takes a single
 .Fa void *
 argument.
 Upon invocation,
@@ -126,267 +361,57 @@ Upon invocation,
 will receive
 .Fa arg
 as its only argument.
-The return value from
-.Fn timeout
-is a
-.Ft struct callout_handle
-which can be used in conjunction with the
-.Fn untimeout
-function to request that a scheduled timeout be canceled.
 The
-.Fn timeout
-call is the old style and new code should use the
-.Fn callout_*
-functions.
-.Pp
-The function
-.Fn callout_handle_init
-can be used to initialize a handle to a state which will cause
-any calls to
-.Fn untimeout
-with that handle to return with no side
-effects.
-.Pp
-Assigning a callout handle the value of
-.Fn CALLOUT_HANDLE_INITIALIZER
-performs the same function as
-.Fn callout_handle_init
-and is provided for use on statically declared or global callout handles.
-.Pp
-The function
-.Fn untimeout
-cancels the timeout associated with
-.Fa handle
-using the
+.Fn callout_schedule
+functions reuse the
 .Fa func
 and
 .Fa arg
-arguments to validate the handle.
-If the handle does not correspond to a timeout with
-the function
-.Fa func
-taking the argument
-.Fa arg
-no action is taken.
-.Fa handle
-must be initialized by a previous call to
-.Fn timeout ,
-.Fn callout_handle_init ,
-or assigned the value of
-.Fn CALLOUT_HANDLE_INITIALIZER "&handle"
-before being passed to
-.Fn untimeout .
-The behavior of calling
-.Fn untimeout
-with an uninitialized handle
-is undefined.
-The
-.Fn untimeout
-call is the old style and new code should use the
-.Fn callout_*
-functions.
-.Pp
-As handles are recycled by the system, it is possible (although unlikely)
-that a handle from one invocation of
-.Fn timeout
-may match the handle of another invocation of
-.Fn 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
-.Fn timeout
-and
-.Fn untimeout .
-Timeouts are executed from
-.Fn softclock
-with the
-.Va Giant
-lock held.
-Thus they are protected from re-entrancy.
-.Pp
-The functions
-.Fn callout_init ,
-.Fn callout_init_mtx ,
-.Fn callout_init_rm ,
-.Fn callout_init_rw ,
-.Fn callout_stop ,
-.Fn callout_drain ,
+arguments from the previous callout.
+Note that one of the
 .Fn callout_reset
-and
-.Fn callout_schedule
-are low-level routines for clients who wish to allocate their own
-callout structures.
-.Pp
-The function
-.Fn callout_init
-initializes a callout so it can be passed to
-.Fn callout_stop ,
-.Fn callout_drain ,
-.Fn callout_reset
-or
-.Fn callout_schedule
-without any side effects.
-If the
-.Fa mpsafe
-argument is zero,
-the callout structure is not considered to be
-.Dq multi-processor safe ;
-that is,
-the Giant lock will be acquired before calling the callout function,
-and released when the callout function returns.
-.Pp
-The
-.Fn callout_init_mtx
-function may be used as an alternative to
-.Fn callout_init .
-The parameter
-.Fa 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
-.Fa flags
-may be specified:
-.Bl -tag -width ".Dv CALLOUT_RETURNUNLOCKED"
-.It Dv CALLOUT_RETURNUNLOCKED
-The callout function will release
-.Fa mtx
-itself, so the callout subsystem should not attempt to unlock it
-after the callout function returns.
-.El
-.Pp
-The
-.Fn callout_init_rw
-and the
-.Fn callout_init_rm
-fuctions serve the need of using rwlocks and rmlocks in conjunction
-with callouts.
-The functions do the same as
-.Fn callout_init
-with the possibility of specifying an extra
-.Fa rw
-or
-.Fa rm
-argument.
-If an
-.Fa rm
-argument is specified, the lock should be created without passing the
-.Dv RM_SLEEPABLE
-flag.
-The usable lock classes are currently limited to mutexes, rwlocks and
-non-sleepable rmlocks, because callout handlers run in softclock swi,
-so they cannot sleep nor acquire sleepable locks like sx or lockmgr.
-The following
-.Fa flags
-may be specified:
-.Bl -tag -width ".Dv CALLOUT_SHAREDLOCK"
-.It Dv CALLOUT_SHAREDLOCK
-The lock is only acquired in read mode when running the callout handler.
-It has no effects when used in conjunction with
-.Fa mtx .
-.El
-.Pp
-The function
-.Fn callout_stop
-cancels a callout if it is currently pending.
-If the callout is pending, then
-.Fn 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.
-.Pp
-The function
-.Fn callout_drain
-is identical to
-.Fn 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
-.Fn callout_drain .
-However, the callout subsystem does guarantee that the callout will be
-fully stopped before
-.Fn callout_drain
-returns.
-.Pp
-The function
-.Fn callout_reset
-first performs the equivalent of
-.Fn callout_stop
-to disestablish the callout, and then establishes a new callout in the
-same manner as
-.Fn timeout .
-If there was already a pending callout and it was rescheduled, then
-.Fn 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
-.Fn callout_schedule
-(re)schedules an existing callout for a new period of time;
-it is equivalent to calling
-.Fn callout_reset
-with the
+functions must always be called to initialize
 .Fa func
 and
 .Fa arg
-parameters extracted from the callout structure (though possibly with
-lower overhead).
+before one of the
+.Fn callout_schedule
+functions can be used.
 .Pp
-The functions
-.Fn callout_reset_on
+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
+.Fn callout_reset_on ,
+.Fn callout_reset_sbt_on ,
 and
 .Fn callout_schedule_on
-are equivalent to
-.Fn callout_reset
-and
-.Fn callout_schedule
-but take an extra parameter specifying the target CPU for the callout.
-.Pp
-The function
-.Fn callout_reset_sbt_on
-allows to get higher time resolution, taking relative or absolute time
-and precision instead of relative ticks count.
-If specified time is in past, it will be silently converted to present
-to run handler as soon as possible.
-.Pp
-The following
-.Fa flags
-may be specified:
-.Bl -tag -width ".Dv C_DIRECT_EXEC"
-.It Dv C_ABSOLUTE
-Handle the
-.Fa sbt
-argument as absolute time of the event since boot, or relative time otherwise.
-.It Dv C_DIRECT_EXEC
-Run handler directly from hardware interrupt context instead of softclock swi.
-It is faster, but puts more constraints on handlers.
-Handlers may use only spin mutexes for locking, and they must be fast because
-they run with absolute priority.
-.It Fn 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.
-Smaller value allows to aggregate more events in one timer interrupt to
-reduce processing overhead and power consumption.
-.It Dv C_HARDCLOCK
-Align the timeouts, if possible, to 
-.Fn hardclock  
-calls.
-.El
-.Pp
-The functions
-.Fn callout_reset_curcpu
+functions assign the callout to CPU
+.Fa cpu .
+The
+.Fn callout_reset_curcpu ,
+.Fn callout_reset_sbt_curpu ,
 and
 .Fn callout_schedule_curcpu
-are wrappers for
-.Fn callout_reset_on
+functions assign the callout to the current CPU.
+The
+.Fn callout_reset ,
+.Fn callout_reset_sbt ,
 and
-.Fn callout_schedule_on
-using the current CPU as the target CPU.
+.Fn callout_schedule
+functions schedule the callout to execute in the softclock thread of the CPU
+to which it is currently assigned.
+.Pp
+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
+.Va 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
+.Va kern.pin_pcpu_swi
+loader tunable to a non-zero value.
 .Pp
 The macros
 .Fn callout_pending ,
@@ -394,10 +419,6 @@ The macros
 and
 .Fn 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
-.Sx "Avoiding Race Conditions"
-below for further details.
 The
 .Fn callout_pending
 macro checks whether a callout is
@@ -410,7 +431,8 @@ starts to process this callout,
 .Fn callout_pending
 will return
 .Dv FALSE
-even though the callout function may not have finished (or even begun)
+even though the callout function may not have finished
+.Pq or even begun
 executing.
 The
 .Fn callout_active
@@ -434,45 +456,64 @@ but it
 clear it when a callout expires normally via the execution of the
 callout function.
 .Ss "Avoiding Race Conditions"
-The callout subsystem invokes callout functions from its own timer
+The callout subsystem invokes callout functions from its own thread
 context.
-Without some kind of synchronization it is possible that a callout
+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
+In particular, since callout functions typically acquire a lock 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
+but is blocked waiting for that lock at the time that another thread
 tries to reset or stop the callout.
 .Pp
-The callout subsystem provides a number of mechanisms to address these
-synchronization concerns:
+There are three main techniques for addressing these
+synchronization concerns.
+The first approach is preferred as it is the simplest:
 .Bl -enum -offset indent
 .It
-If the callout has an associated mutex that was specified using the
-.Fn callout_init_mtx
-function (or implicitly specified as the
-.Va Giant
-mutex using
+Callouts can be associated with a specific lock when they are initialized
+by
+.Fn callout_init_mtx ,
+.Fn callout_init_rm ,
+or
+.Fn 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
+.Fn callout_stop
+or one of the
+.Fn callout_reset
+or
+.Fn callout_schedule
+functions to provide this safety.
+.Pp
+A callout initialized via
 .Fn callout_init
 with
 .Fa mpsafe
-set to
-.Dv FALSE ) ,
-then this mutex is used to avoid the race conditions.
-The associated mutex must be acquired by the caller before calling
-.Fn callout_stop
-or
-.Fn 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
-.Fn callout_drain
-before destroying the callout or its associated mutex.
+set to zero is implicitly associated with the
+.Va Giant
+mutex.
+If
+.Va Giant
+is held when cancelling or rescheduling the callout,
+then its use will prevent races with the callout function.
 .It
 The return value from
 .Fn callout_stop
-and
+.Po
+or the
 .Fn callout_reset
+and
+.Fn callout_schedule
+function families
+.Pc
 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
@@ -607,19 +648,134 @@ data objects that have already been destroyed or recycled.
 To ensure that the callout is completely finished, a call to
 .Fn 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.
+.Sh LEGACY API
+.Bf Sy
+The functions below are a legacy API that will be removed in a future release.
+New code should not use these routines.
+.Ef
+.Pp
+The function
+.Fn timeout
+schedules a call to the function given by the argument
+.Fa func
+to take place after
+.Fa ticks Ns No /hz
+seconds.
+Non-positive values of
+.Fa ticks
+are silently converted to the value
+.Sq 1 .
+.Fa func
+should be a pointer to a function that takes a
+.Fa void *
+argument.
+Upon invocation,
+.Fa func
+will receive
+.Fa arg
+as its only argument.
+The return value from
+.Fn timeout
+is a
+.Ft struct callout_handle
+which can be used in conjunction with the
+.Fn untimeout
+function to request that a scheduled timeout be canceled.
+.Pp
+The function
+.Fn callout_handle_init
+can be used to initialize a handle to a state which will cause
+any calls to
+.Fn untimeout
+with that handle to return with no side
+effects.
+.Pp
+Assigning a callout handle the value of
+.Fn CALLOUT_HANDLE_INITIALIZER
+performs the same function as
+.Fn callout_handle_init
+and is provided for use on statically declared or global callout handles.
+.Pp
+The function
+.Fn untimeout
+cancels the timeout associated with
+.Fa handle
+using the
+.Fa func
+and
+.Fa arg
+arguments to validate the handle.
+If the handle does not correspond to a timeout with
+the function
+.Fa func
+taking the argument
+.Fa arg
+no action is taken.
+.Fa handle
+must be initialized by a previous call to
+.Fn timeout ,
+.Fn callout_handle_init ,
+or assigned the value of
+.Fn CALLOUT_HANDLE_INITIALIZER "&handle"
+before being passed to
+.Fn untimeout .
+The behavior of calling
+.Fn untimeout
+with an uninitialized handle
+is undefined.
+.Pp
+As handles are recycled by the system, it is possible (although unlikely)
+that a handle from one invocation of
+.Fn timeout
+may match the handle of another invocation of
+.Fn 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
+.Fn timeout
+and
+.Fn untimeout .
+Timeouts are executed from
+.Fn softclock
+with the
+.Va Giant
+lock held.
+Thus they are protected from re-entrancy.
 .Sh RETURN VALUES
 The
-.Fn timeout
-function returns a
-.Ft struct callout_handle
-that can be passed to
-.Fn untimeout .
+.Fn callout_active
+macro returns the state of a callout's
+.Em active
+flag.
+.Pp
+The
+.Fn callout_pending
+macro returns the state of a callout's
+.Em pending
+flag.
+.Pp
+The
+.Fn callout_reset
+and
+.Fn callout_schedule
+function families return non-zero if the callout was pending before the new
+function invocation was scheduled.
+.Pp
 The
 .Fn callout_stop
 and
 .Fn callout_drain
 functions return non-zero if the callout was still pending when it was
 called or zero otherwise.
+The
+.Fn timeout
+function returns a
+.Ft struct callout_handle
+that can be passed to
+.Fn untimeout .
 .Sh HISTORY
 The current timeout and untimeout routines are based on the work of
 .An Adam M. Costello