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CONDVAR(9) Kernel Developer's Manual CONDVAR(9)

cv, condvar, cv_init, cv_destroy, cv_wait, cv_wait_sig, cv_timedwait, cv_timedwait_sig, cv_timedwaitbt, cv_timedwaitbt_sig, cv_signal, cv_broadcast, cv_has_waiters — condition variables

#include <sys/condvar.h>

void
cv_init(kcondvar_t *cv, const char *wmesg);

void
cv_destroy(kcondvar_t *cv);

void
cv_wait(kcondvar_t *cv, kmutex_t *mtx);

int
cv_wait_sig(kcondvar_t *cv, kmutex_t *mtx);

int
cv_timedwait(kcondvar_t *cv, kmutex_t *mtx, int ticks);

int
cv_timedwait_sig(kcondvar_t *cv, kmutex_t *mtx, int ticks);

int
cv_timedwaitbt(kcondvar_t *cv, kmutex_t *mtx, struct bintime *bt, const struct bintime *epsilon);

int
cv_timedwaitbt_sig(kcondvar_t *cv, kmutex_t *mtx, struct bintime *bt, const struct bintime *epsilon);

void
cv_signal(kcondvar_t *cv);

void
cv_broadcast(kcondvar_t *cv);

bool
cv_has_waiters(kcondvar_t *cv);


options DIAGNOSTIC
options LOCKDEBUG

Condition variables (CVs) are used in the kernel to synchronize access to resources that are limited (for example, memory) and to wait for pending I/O operations to complete.

The kcondvar_t type provides storage for the CV object. This should be treated as an opaque object and not examined directly by consumers.

options DIAGNOSTIC

Kernels compiled with the DIAGNOSTIC option perform basic sanity checks on CV operations.

options LOCKDEBUG

Kernels compiled with the LOCKDEBUG option perform potentially CPU intensive sanity checks on CV operations.

(cv, wmesg)

Initialize a CV for use. No other operations can be performed on the CV until it has been initialized.

The wmesg argument specifies a string of no more than 8 characters that describes the resource or condition associated with the CV. The kernel does not use this argument directly but makes it available for utilities such as ps(1) to display.

(cv)

Release resources used by a CV. The CV must not be in use when it is destroyed, and must not be used afterwards.

(cv, mtx)

Cause the current LWP to wait non-interruptably for access to a resource, or for an I/O operation to complete. The LWP will resume execution when awoken by another thread using () or cv_broadcast().

mtx specifies a kernel mutex to be used as an interlock, and must be held by the calling LWP on entry to (). It will be released once the LWP has prepared to sleep, and will be reacquired before cv_wait() returns.

A small window exists between testing for availability of a resource and waiting for the resource with (), in which the resource may become available again. The interlock is used to guarantee that the resource will not be signalled as available until the calling LWP has begun to wait for it.

Non-interruptable waits have the potential to deadlock the system, and so must be kept short (typically, under one second).

(cv, mtx)

As per (), but causes the current LWP to wait interruptably. If the LWP receives a signal, or is interrupted by another condition such as its containing process exiting, the wait is ended early and an error code returned.

If () returns as a result of a signal, the return value is ERESTART if the signal has the SA_RESTART property. If awoken normally, the value is zero, and EINTR under all other conditions.

cv_timedwait(cv, mtx, ticks)

As per (), but will return early if a timeout specified by the ticks argument expires.

ticks is an architecture and system dependent value related to the number of clock interrupts per second. See hz(9) for details. The mstohz(9) macro can be used to convert a timeout expressed in milliseconds to one suitable for (). If the ticks argument is zero, cv_timedwait() behaves exactly like cv_wait().

If the timeout expires before the LWP is awoken, the return value is EWOULDBLOCK. If awoken normally, the return value is zero.

(cv, mtx, ticks)

As per (), but also accepts a timeout value and will return EWOULDBLOCK if the timeout expires.

(cv, mtx, bt, epsilon)
 
(cv, mtx, bt, epsilon)

Similar to () and cv_timedwait_sig(), but bt is decremented in place with the amount of time actually waited, and on return contains the amount of time remaining, possibly negative if the timeout expired.

The hint epsilon requests that the wakeup not be delayed more than bt + epsilon, so that the system can coalesce multiple wakeups within their respective epsilons into a single high-resolution clock interrupt or choose to use cheaper low-resolution clock interrupts instead.

However, the system is still limited by its best clock interrupt resolution and by scheduling competition, which may delay the wakeup by more than bt + epsilon.

(cv)

Awaken one LWP (potentially among many) that is waiting on the specified condition variable. The mutex passed to the wait function (mtx) must also be held when calling ().

(Note that () is erroneously named in that it does not send a signal in the traditional sense to LWPs waiting on a CV.)

cv_broadcast(cv)

Awaken all LWPs waiting on the specified condition variable. The mutex passed to the wait function (mtx) must also be held when calling ().

cv_has_waiters(cv)

Return true if one or more LWPs are waiting on the specified condition variable.

() cannot test reliably for interruptable waits. It should only be used to test for non-interruptable waits made using cv_wait().

() should only be used when making diagnostic assertions, and must be called while holding the interlocking mutex passed to cv_wait().

Consuming a resource:

	/*
	 * Lock the resource.  Its mutex will also serve as the
	 * interlock.
	 */
	mutex_enter(&res->mutex);

	/*
	 * Wait for the resource to become available.  Timeout after
	 * five seconds.  If the resource is not available within the
	 * alloted time, return an error.
	 */
	struct bintime timeout = { .sec = 5, .frac = 0 };
	const struct bintime epsilon = { .sec = 1, .frac = 0 };
	while (res->state == BUSY) {
		error = cv_timedwaitbt(&res->condvar, \
		    &res->mutex, &timeout, &epsilon);
		if (error) {
			KASSERT(error == EWOULDBLOCK);
			if (res->state != BUSY)
				break;
			mutex_exit(&res->mutex);
			return ETIMEDOUT;
		}
	}

	/*
	 * It's now available to us.  Take ownership of the
	 * resource, and consume it.
	 */
	res->state = BUSY;
	mutex_exit(&res->mutex);
	consume(res);

Releasing a resource for the next consumer to use:

	mutex_enter(&res->mutex);
	res->state = IDLE;
	cv_signal(&res->condvar);
	mutex_exit(&res->mutex);

The core of the CV implementation is in sys/kern/kern_condvar.c.

The header file sys/sys/condvar.h describes the public interface.

sigaction(2), membar_ops(3), errno(9), mstohz(9), mutex(9), rwlock(9)

Jim Mauro and Richard McDougall, Solaris Internals: Core Kernel Architecture, Prentice Hall, 2001, ISBN 0-13-022496-0.

The CV primitives first appeared in NetBSD 5.0. The cv_timedwaitbt() and cv_timedwaitbt_sig() primitives first appeared in NetBSD 9.0.

November 12, 2017 NetBSD-9.2