NAME

pcq — producer/consumer queue

SYNOPSIS

#include <sys/pcq.h>

pcq_t *
pcq_create(size_t maxlen, km_flags_t kmflags);

void
pcq_destroy(pcq_t *pcq);

void *
pcq_get(pcq_t *pcq);

size_t
pcq_maxitems(pcq_t *pcq);

void *
pcq_peek(pcq_t *pcq);

bool
pcq_put(pcq_t *pcq, void *item);

DESCRIPTION

The machine-independent pcq interface provides lockless producer/consumer queues. A queue (pcq_t) allows multiple writers (producers), but only a single reader (consumer). The consumer is expected to be protected by a lock that covers the structure that the pcq_t is embedded into (e.g., socket lock, ifnet hwlock). These queues operate in a first-in, first-out (FIFO) manner. The act of inserting or removing an item from a pcq_t does not modify the item in any way. pcq does not prevent an item from being inserted multiple times into a single pcq_t.

FUNCTIONS

pcq_create(maxlen, kmflags): Create a queue that can store at most maxlen items at one time. kmflags should be either KM_SLEEP, if pcq_create() is allowed to sleep until resources are available, or KM_NOSLEEP if it should return NULL immediately, if resources are unavailable.
pcq_destroy(pcq): Free the resources held by pcq.
pcq_get(pcq): Remove the next item to be consumed from the queue and return it. If the queue is empty, return NULL. The caller must prevent concurrent gets from occurring.
pcq_maxitems(pcq): Return the maximum number of items that the queue can store at any one time.
pcq_peek(pcq): Return the next item to be consumed from the queue but do not remove it from the queue. If the queue is empty, return NULL.
pcq_put(pcq, item): Place an item at the end of the queue. If there is no room in the queue for the item, return false; otherwise, return true. The item must not have the value of NULL.

Memory ordering

Any memory operations sequenced before pcq_put() of an item in one thread happen before all memory operations with data dependencies on the item returned by pcq_get() or pcq_peek() in another thread. For example:

int mumble;

/* producer */
mumble = 42;			// A
foo->x = 123;			// B
refcnt = foo->refcnt;		// C
pcq_put(pcq, foo);
KASSERT(refcnt == 0);

/* consumer */
foo = pcq_get(pcq);
if (foo == NULL)
	return;
atomic_inc_uint(&foo->refcnt);	// D
x = foo->x;			// E
if (x == 123)
	KASSERT(mumble == 42);	// F

In this example, memory operations B and C happen-before D and E. However, no ordering is guaranteed for A or F relative to any other memory operations, because the memory location of mumble is independent of the pointer foo returned by pcq_get().

If you must guarantee A happens before F, then on the consumer side, after pcq_get() or pcq_peek(), you can call membar_acquire() to turn it into an acquire operation instead of a consume operation; pcq_put() serves as the matching release operation. (This is a little dicey. Perhaps there should be separate pcq_peek_acq() and pcq_get_acq() operations if this semantics is necessary.)

CODE REFERENCES

The pcq interface is implemented within the file sys/kern/subr_pcq.c.

HISTORY

The pcq interface first appeared in NetBSD 6.0.