CALLOUT(9) | Kernel Developer's Manual | CALLOUT(9) |
callout_init
, callout_destroy
,
callout_halt
, callout_reset
,
callout_schedule
,
callout_setfunc
, callout_stop
,
callout_pending
,
callout_expired
,
callout_invoking
, callout_ack
—
#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);
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
().
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
().
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.
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.
January 12, 2020 | NetBSD 9.0 |