PCQ(9)

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.

SEE ALSO
atomic_ops(3), queue(3)

HISTORY
The pcq interface first appeared in NetBSD 6.0.