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condvar(9f)
condvar(9F) Kernel Functions condvar(9F)
NAME
condvar, cv_init, cv_destroy, cv_wait, cv_signal, cv_broadcast,
cv_wait_sig, cv_timedwait, cv_timedwait_sig, cv_reltimedwait, cv_rel‐
timedwait_sig - condition variable routines
SYNOPSIS
#include <sys/ksynch.h>
void cv_init(kcondvar_t *cvp, char *name, kcv_type_t type, void *arg);
void cv_destroy(kcondvar_t *cvp);
void cv_wait(kcondvar_t *cvp, kmutex_t *mp);
void cv_signal(kcondvar_t *cvp);
void cv_broadcast(kcondvar_t *cvp);
int cv_wait_sig(kcondvar_t *cvp, kmutex_t *mp);
clock_t cv_timedwait(kcondvar_t *cvp, kmutex_t *mp, clock_t timeout);
clock_t cv_timedwait_sig(kcondvar_t *cvp, kmutex_t *mp, clock_t timeout);
clock_t cv_reltimedwait(kcondvar_t *cvp, kmutex_t *mp, clock_t delta,
time_res_t resolution);
clock_t cv_reltimedwait_sig(kcondvar_t *cvp, kmutex_t *mp, clock_t delta,
time_res_t resolution);
INTERFACE LEVEL
Solaris DDI specific (Solaris DDI).
PARAMETERS
cvp A pointer to an abstract data type kcondvar_t.
mp A pointer to a mutual exclusion lock (kmutex_t), initial‐
ized by mutex_init(9F) and held by the caller.
name Descriptive string. This is obsolete and should be NULL.
(Non-NULL strings are legal, but they're a waste of ker‐
nel memory.)
type The constant CV_DRIVER.
arg A type-specific argument, drivers should pass arg as
NULL.
timeout A time, in absolute ticks since boot, when cv_timedwait()
or cv_timedwait_sig() should return.
delta A time, in relative ticks, when cv_reltimedwait() or
cv_reltimedwait_sig() should return.
resolution A flag that specifies how accurate the relative time in‐
terval should be. Possible values are TR_NANOSEC, TR_MI‐
CROSEC, TR_MILLISEC, TR_SEC, or TR_CLOCK_TICK.
TR_CLOCK_TICK indicates that the interval should be
aligned to system clock ticks. This information is used
to anticipate or defer the timeout expiration in order to
batch process similarly expiring events, allowing the
system to stay idle for longer periods of time and en‐
hance its power efficiency.
DESCRIPTION
Condition variables are a standard form of thread synchronization. They
are designed to be used with mutual exclusion locks (mutexes). The as‐
sociated mutex is used to ensure that a condition can be checked atomi‐
cally and that the thread can block on the associated condition vari‐
able without missing either a change to the condition or a signal that
the condition has changed. Condition variables must be initialized by
calling cv_init(), and must be deallocated by calling cv_destroy().
The usual use of condition variables is to check a condition (for exam‐
ple, device state, data structure reference count, etc.) while holding
a mutex which keeps other threads from changing the condition. If the
condition is such that the thread should block, cv_wait() is called
with a related condition variable and the mutex. At some later point in
time, another thread would acquire the mutex, set the condition such
that the previous thread can be unblocked, unblock the previous thread
with cv_signal() or cv_broadcast(), and then release the mutex.
The cv_wait() function suspends the calling thread and exits the mutex
atomically so that another thread which holds the mutex cannot signal
on the condition variable until the blocking thread is blocked. Before
returning, the mutex is reacquired.
The cv_signal() function signals the condition and wakes one blocked
thread. All blocked threads can be unblocked by calling cv_broadcast().
cv_signal() and cv_broadcast() can be called by a thread even if it
does not hold the mutex passed into cv_wait(), though holding the mutex
is necessary to ensure predictable scheduling.
The cv_wait_sig() function is similar to cv_wait() but returns 0 if a
signal (for example, by kill(2)) is sent to the thread. In any case,
the mutex is reacquired before returning.
The cv_timedwait() function is similar to cv_wait(), except that it re‐
turns −1 without the condition being signaled after the timeout time
has been reached.
The cv_timedwait_sig() function is similar to cv_timedwait() and
cv_wait_sig(), except that it returns −1 without the condition being
signaled after the timeout time has been reached, or 0 if a signal (for
example, by kill(2)) is sent to the thread.
For both cv_timedwait() and cv_timedwait_sig(), time is in absolute
clock ticks since the last system reboot. The current time may be found
by calling ddi_get_lbolt(9F).
The cv_reltimedwait() function is similar to cv_timedwait(), except
that it takes a relative time value as argument and it also takes an
additional argument to specify the accuracy of such interval. The
cv_reltimedwait_sig() function is analogous to cv_timedwait_sig() but
takes the same arguments as cv_reltimedwait().
RETURN VALUES
0 For cv_wait_sig(), cv_timedwait_sig(), and cv_reltimed‐
wait_sig(), this value indicates that the condition was not
necessarily signaled and the function returned because a sig‐
nal (as in kill(2)) was pending.
−1 For cv_timedwait(), cv_timedwait_sig(), cv_reltimedwait(), and
cv_reltimedwait_sig(), this value indicates that the condition
was not necessarily signaled and the function returned because
the timeout time was reached.
>0 For cv_wait_sig(), cv_timedwait(), cv_timedwait_sig(), cv_rel‐
timedwait(), and cv_reltimedwait_sig(), a value greater than 0
indicates that the condition was met and the function returned
due to a call to either cv_signal() or cv_broadcast(), or due
to a premature wakeup (see NOTES).
CONTEXT
These functions can be called from user, kernel or interrupt context.
In most cases, however, cv_wait(), cv_timedwait(), cv_wait_sig(),
cv_timedwait_sig(), cv_reltimedwait(), and cv_reltimedwait_sig() should
not be called from interrupt context, and cannot be called from a high-
level interrupt context.
If cv_wait(), cv_timedwait(), cv_wait_sig(), cv_timedwait_sig(),
cv_reltimedwait(), or cv_reltimedwait_sig() are used from interrupt
context, lower-priority interrupts will not be serviced during the
wait. This means that if the thread that will eventually perform the
wakeup becomes blocked on anything that requires the lower-priority in‐
terrupt, the system will hang.
For example, the thread that will perform the wakeup may need to first
allocate memory. This memory allocation may require waiting for paging
I/O to complete, which may require a lower-priority disk or network in‐
terrupt to be serviced. In general, situations like this are hard to
predict, so it is advisable to avoid waiting on condition variables or
semaphores in an interrupt context.
EXAMPLES
Example 1 Waiting for a Flag Value in a Driver's Unit
Here the condition being waited for is a flag value in a driver's unit
structure. The condition variable is also in the unit structure, and
the flag word is protected by a mutex in the unit structure.
mutex_enter(&un->un_lock);
while (un->un_flag & UNIT_BUSY)
cv_wait(&un->un_cv, &un->un_lock);
un->un_flag |= UNIT_BUSY;
mutex_exit(&un->un_lock);
Example 2 Unblocking Threads Blocked by the Code in Example 1
At some later point in time, another thread would execute the following
to unblock any threads blocked by the above code.
mutex_enter(&un->un_lock);
un->un_flag &= ~UNIT_BUSY;
cv_broadcast(&un->un_cv);
mutex_exit(&un->un_lock);
NOTES
It is possible for cv_wait(), cv_wait_sig(), cv_timedwait(), cv_timed‐
wait_sig(), cv_reltimedwait(), and cv_reltimedwait_sig() to return pre‐
maturely, that is, not due to a call to cv_signal() or cv_broadcast().
This occurs most commonly in the case of cv_wait_sig() and cv_timed‐
wait_sig() and cv_timedwait_sig() when the thread is stopped and
restarted by job control signals or by a debugger, but can happen in
other cases as well, even for cv_wait(). Code that calls these func‐
tions must always recheck the reason for blocking and call again if the
reason for blocking is still true.
If your driver needs to wait on behalf of processes that have real-time
constraints, use cv_timedwait() or cv_reltimedwait() rather than de‐
lay(9F). The delay() function calls timeout(9F), which can be subject
to priority inversions.
Not all threads can receive signals from user level processes. In cases
where such reception is impossible (such as during execution of
close(9E) due to exit(2)), cv_wait_sig() behaves as cv_wait(),
cv_timedwait_sig() behaves as cv_timedwait(), and cv_reltimedwait_sig()
behaves as cv_reltimedwait(). To avoid unkillable processes, users of
these functions may need to protect against waiting indefinitely for
events that might not occur. The ddi_can_receive_sig(9F) function is
provided to detect when signal reception is possible.
SEE ALSO
kill(2), ddi_can_receive_sig(9F), ddi_get_lbolt(9F), mutex(9F), mu‐
tex_init(9F)
Writing Device Drivers in Oracle Solaris 11.4
Oracle Solaris 11.4 13 Nov 2009 condvar(9F)