SEMAPHORE(9F) Kernel Functions for Drivers SEMAPHORE(9F)

semaphore, sema_init, sema_destroy, sema_p, sema_p_sig, sema_v, sema_tryp —
semaphore functions

#include <sys/ksynch.h>

void
sema_init(ksema_t *sp, uint_t val, char *name, ksema_type_t type, void *arg);

void
sema_destroy(ksema_t *sp);

void
sema_p(ksema_t *sp);

void
sema_v(ksema_t *sp);

int
sema_p_sig(ksema_t *sp);

int
sema_tryp(ksema_t *sp);

illumos DDI specific (illumos DDI).

sp
A pointer to a semaphore, type ksema_t.
val
Initial value for semaphore.
name
Descriptive string. This is obsolete and should be NULL. (Non-NULL strings are legal, but they are a waste of kernel memory.)
type
Variant type of the semaphore. Currently, only SEMA_DRIVER is supported.
arg
Type-specific argument; should be NULL.

These functions implement counting semaphores as described by Dijkstra. A semaphore has a value which is atomically decremented by sema_p() and atomically incremented by sema_v(). The value must always be greater than or equal to zero. If sema_p() is called and the value is zero, the calling thread is blocked until another thread performs a sema_v() operation on the semaphore.

Semaphores are initialized by calling sema_init(). The argument, val, gives the initial value for the semaphore. The semaphore storage is provided by the caller but more may be dynamically allocated, if necessary, by sema_init(). For this reason, sema_destroy() should be called before deallocating the storage containing the semaphore.

The sema_p_sig() function decrements the semaphore, as does sema_p(). However, if the semaphore value is zero, sema_p_sig() will return without decrementing the value if a signal (that is, from kill(2)) is pending for the thread.

The sema_tryp() function will decrement the semaphore value only if it is greater than zero, and will not block.

These functions can be called from user, interrupt, or kernel context, except for sema_init() and sema_destroy(), which can be called from user or kernel context only. None of these functions can be called from a high-level interrupt context. In most cases, sema_v() and sema_p() should not be called from any interrupt context.

If sema_p() is 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 sema_v() becomes blocked on anything that requires the lower-priority interrupt, the system will hang.

For example, the thread that will perform the sema_v() 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 interrupt to be serviced. In general, situations like this are hard to predict, so it is advisable to avoid waiting on semaphores or condition variables in an interrupt context.

Similar to many other synchronization mechanisms, semaphores should not be used in any code path that requires synchronization while handling system panic, at which time many of the semaphore operations become no-ops.

sema_tryp() could not decrement the semaphore value because it was zero.
sema_p_sig() was not able to decrement the semaphore value and detected a pending signal.

kill(2), condvar(9F), mutex(9F)

Writing Device Drivers

July 30, 2018 OmniOS