callout_pending (9)

Leading comments

	$NetBSD: timeout.9,v 1.2 1996/06/23 22:32:34 pk Exp $

 Copyright (c) 1996 The NetBSD Foundation, Inc.
 All rights reserved.

 This code is derived from software contributed to The NetBSD Foundation
 by Paul Kranenburg.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 1. Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaim...

(The comments found at the beginning of the groff file "man9/callout_pending.9freebsd".)

NAME

timeout untimeout callout_handle_init callout_init callout_init_mtx callout_init_rm callout_init_rw callout_stop callout_drain callout_reset callout_reset_on callout_reset_curcpu callout_reset_sbt callout_reset_sbt_on callout_reset_sbt_curcpu callout_schedule callout_schedule_on callout_schedule_curcpu callout_pending callout_active callout_deactivate - execute a function after a specified length of time

SYNOPSIS

In sys/types.h In sys/systm.h

typedef void timeout_t (void *);

Ft struct callout_handle Fn timeout timeout_t *func void *arg int ticks Ft void Fn callout_handle_init struct callout_handle *handle

struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle);

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 Ft void Fn callout_init_rm struct callout *c struct rmlock *rm int flags 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 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 Ft int Fn callout_schedule_on struct callout *c int ticks int cpu Ft int Fn callout_schedule_curcpu struct callout *c int ticks Ft int Fn callout_pending struct callout *c Ft int Fn callout_active struct callout *c Ft void Fn callout_deactivate struct callout *c

DESCRIPTION

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 `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. The Fn timeout call is the old style and new code should use the Fn callout_* functions.

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.

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.

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. The Fn untimeout call is the old style and new code should use the Fn callout_* functions.

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 Giant lock held. Thus they are protected from re-entrancy.

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

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 ``multi-processor safe'' that is, the Giant lock will be acquired before calling the callout function, and released when the callout function returns.

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:

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.

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 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:

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 .

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.

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.

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 Fa func and Fa arg parameters extracted from the callout structure (though possibly with lower overhead).

The functions Fn callout_reset_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.

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.

The following Fa flags may be specified:

C_ABSOLUTE

Handle the Fa sbt argument as absolute time of the event since boot, or relative time otherwise.

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.

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.

The functions Fn callout_reset_curcpu and Fn callout_schedule_curcpu are wrappers for Fn callout_reset_on and Fn callout_schedule_on using the current CPU as the target CPU.

The macros Fn callout_pending , Fn callout_active 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 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, Fn callout_pending will return FALSE even though the callout function may not have finished (or even begun) executing. The Fn callout_active macro checks whether a callout is marked as active and the Fn 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 Fn callout_stop and Fn callout_drain , but it does not clear it when a callout expires normally via the execution of the callout function.

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 Fn callout_init_mtx function (or implicitly specified as the Giant mutex using Fn callout_init with Fa 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 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.
2. The return value from Fn callout_stop and Fn 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 Fn callout_pending , Fn callout_active and Fn 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 Fn callout_deactivate macro. The Fn callout_stop and Fn 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 Fn callout_pending returns TRUE This indicates that the callout was rescheduled using Fn callout_reset just before the callout function was invoked. If Fn 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 Fn 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 Fn callout_stop and Fn 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 Fn callout_deactivate call.

   The above technique additionally ensures that the active flag always reflects whether the callout is effectively enabled or disabled. If Fn 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 Fn callout_drain should be used.

RETURN VALUES

The Fn timeout function returns a Ft struct callout_handle that can be passed to Fn untimeout . 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.

HISTORY

The current timeout and untimeout routines are based on the work of An Adam M. Costello and An George Varghese , published in a technical report entitled "Redesigning the BSD Callout and Timer Facilities" and modified slightly for inclusion in Fx by An Justin T. Gibbs . The original work on the data structures used in this implementation was published by An G. Varghese and An 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.