NAME
callout_init
,
callout_destroy
,
callout_halt
, callout_reset
,
callout_schedule
,
callout_setfunc
,
callout_stop
,
callout_pending
,
callout_expired
,
callout_invoking
,
callout_ack
—
execute a function after a specified
length of time
SYNOPSIS
#include
<sys/callout.h>
void
callout_init
(callout_t
*c, u_int
flags);
void
callout_destroy
(callout_t
*c);
void
callout_reset
(callout_t
*c, int ticks,
void (*func)(void *),
void *arg);
void
callout_schedule
(callout_t
*c, int ticks);
void
callout_setfunc
(callout_t
*c, void (*func)(void
*), void *arg);
bool
callout_stop
(callout_t
*c);
bool
callout_halt
(callout_t
*c, void
*interlock);
bool
callout_pending
(callout_t
*c);
bool
callout_expired
(callout_t
*c);
bool
callout_active
(callout_t
*c);
bool
callout_invoking
(callout_t
*c);
void
callout_ack
(callout_t
*c);
DESCRIPTION
The callout
facility provides a mechanism
to execute a function at a given time. The timer is based on the hardclock
timer which ticks hz
times per second. The function
is called at softclock interrupt level.
Clients of the callout
facility are responsible for providing pre-allocated callout structures, or
“handles”. The callout
facility
replaces the historic UNIX functions
timeout
()
and
untimeout
().
FUNCTIONS
The
callout_init
()
function initializes the callout handle c for use. No
operations can be performed on the callout before it is initialized. If the
flags argument is
CALLOUT_MPSAFE
, the handler will be called without
getting the global kernel lock. In this case it should only use functions
that are multiprocessor safe.
callout_destroy
()
destroys the callout, preventing further use. It is provided as a diagnostic
facility intended to catch bugs. To ensure future compatibility,
callout_destroy
() should always be called when the
callout is no longer required (for instance, when a device is being
detached). The callout should be stopped before
callout_destroy
() is called by calling
callout_halt
(). Note that
callout_stop
() shouldn't be used for this
purpose.
The
callout_reset
()
function resets and starts the timer associated with the callout handle
c. When the timer expires after
ticks/hz seconds, the function
specified by func will be called with the argument
arg. If the timer associated with the callout handle
is already running, the callout will simply be rescheduled to execute at the
newly specified time. Once the timer is started, the callout handle is
marked as PENDING. Once the timer expires, the handle is
marked as EXPIRED and INVOKING, and the
PENDING status is cleared.
The
callout_setfunc
()
function sets the function and argument of the callout handle
c to func and
arg respectively. The callout handle must already be
initialized. If a callout will always be used with the same function and
argument, then callout_setfunc
() used in conjunction
with
callout_schedule
()
is slightly more efficient than using
callout_reset
().
The
callout_stop
()
function requests that the timer associated with the callout handle
c be stopped. The PENDING and
EXPIRED status for the callout handle is cleared. It is
safe to call callout_stop
() on a callout handle that
is not pending, so long as it is initialized.
callout_stop
() will return a non-zero value if the
callout was EXPIRED. Note that
callout_stop
() can return while the callout is
running on a different CPU or at a different interrupt priority level on the
current CPU. It can only be said to prevent the callout from firing in the
future, unless explicitly re-scheduled. To stop a callout and wait for
completion, use callout_halt
().
callout_halt
()
acts much like callout_stop
(), but waits for the
callout to complete if it is currently in-flight.
callout_halt
() may not be called from a hard
interrupt handler as it will sleep if the callout is currently executing. If
the callout can take locks (such as mutexes or RW locks), the caller of
callout_halt
() must not hold any of those locks,
otherwise the two could deadlock. To facilitate this,
callout_halt
() can optionally release a single mutex
specified by the interlock parameter. If
interlock is not NULL
and the
calling thread must wait for the callout to complete,
interlock will be released before waiting and
re-acquired before returning. If no wait is required,
interlock will not be released. However, to avoid race
conditions the caller should always assume that
interlock has been released and reacquired, and act
accordingly.
The
callout_pending
()
function tests the PENDING status of the callout handle
c. A PENDING callout is one that has
been started and whose function has not yet been called. Note that it is
possible for a callout's timer to have expired without its function being
called if interrupt level has not dropped low enough to let softclock
interrupts through. Note that it is only safe to test
PENDING status when at softclock interrupt level or
higher.
The
callout_expired
()
function tests to see if the callout's timer has expired and its function
called.
The
callout_active
()
function returns true if a timer has been started but not explicitly
stopped, even if it has already fired.
callout_active
(foo) is
logically the same as
callout_pending
(foo) ||
callout_expired
(foo); it is
implemented as a separate function for compatibility with
FreeBSD and for the special case of
TCP_TIMER_ISARMED
().
Its use is not recommended.
The
callout_invoking
()
function tests the INVOKING status of the callout handle
c. This flag is set just before a callout's function
is being called. Since the priority level is lowered prior to invocation of
the callout function, other pending higher-priority code may run before the
callout function is allowed to run. This may create a race condition if this
higher-priority code deallocates storage containing one or more callout
structures whose callout functions are about to be run. In such cases, one
technique to prevent references to deallocated storage would be to test
whether any callout functions are in the INVOKING state
using callout_invoking
(), and if so, to mark the
data structure and defer storage deallocation until the callout function is
allowed to run. For this handshake protocol to work, the callout function
will have to use the callout_ack
() function to clear
this flag.
The
callout_ack
()
function clears the INVOKING state in the callout handle
c. This is used in situations where it is necessary to
protect against the race condition described under
callout_invoking
().
CONCURRENCY
The callout facility performs locking internally in order to guarantee the atomicity of individual operations performed on callouts. It does not provide life cycle management of user-provided callout data structures, nor does it ensure that groups of operations (multiple function calls) are performed atomically. These aspects of callout management are the responsibility of the user of the callout facility.
Scheduled callouts may be active concurrently in a context different to the user of the callout facility: on another CPU, or at a different interrupt priority level or thread on the current CPU. The callout facility provides only one guarantee in this regard: any given callout will never have multiple concurrent invocations.
SEE ALSO
HISTORY
The callout
facility was implemented by
Artur Grabowski and Thomas Nordin, based on the work of G. Varghese and A.
Lauck, described 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 System Principles,
Austin, Texas, November 1987. It was adapted to the
NetBSD kernel by Jason R. Thorpe.