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

pserialize
passive serialization mechanism

#include <sys/pserialize.h>

pserialize_t
pserialize_create(void);

void
pserialize_destroy(pserialize_t psz);

int
pserialize_read_enter(void);

void
pserialize_read_exit(int s);

void
pserialize_perform(pserialize_t psz);

Passive serialization is a reader / writer synchronisation mechanism designed for lock-less read operations. The read operations may happen from software interrupt at IPL_SOFTCLOCK.

pserialize_create()
Allocate a new synchronisation object.
pserialize_destroy()
Destroy the synchronisation object. No synchronisation activity should happen at this point.
pserialize_read_enter()
Enter the critical path of the reader side. Returns an IPL value, which must be passed to pserialize_read_exit(9). Protected code path is not allowed to block.
pserialize_read_exit()
Exit the critical path of the reader side. Takes the IPL value returned by pserialize_read_enter(9).
pserialize_perform()
Perform the passive serialization on the writer side. Passing of this function ensures that no readers are in action. Writers must be additionally serialized with a separate mechanism, e.g. mutex(9). Operation blocks and it may only be performed from thread context.

Given a global database of frotz records:
	struct frotz {
		...
		struct frotz	*f_next;
	};

	static struct {
		kmutex_t	lock;
		pserialize_t	psz;
		struct frotz	*first;
	} frobbotzim __cacheline_aligned;

Create a frotz and publish it, as a writer:

	struct frotz *f = pool_get(&frotz_pool, PR_WAITOK);

	/* Initialize f.  */
	...

	mutex_enter(&frobbotzim.lock);
	f->f_next = frobbotzim.first;
	/*
	 * Publish the contents of f->f_next before we publish the
	 * pointer to f in frobbotzim.first.
	 */
	membar_producer();
	frobbotzim.first = f;
	mutex_exit(&frobbotzim.lock);

Find a frotz, as a reader:

	struct frotz *f;
	int error = ENOENT;
	int s;

	s = pserialize_read_enter();
	for (f = frobbotzim.first; f != NULL; f = f->f_next) {
		/* Fetch f before we fetch anything f points to.  */
		membar_datadep_consumer();
		if (f->f_... == key) {
			/*
			 * Grab whatever part of the frotz we need.
			 * Note that we can't use the frotz after
			 * pserialize_read_exit, without a stronger
			 * kind of reference, say a reference count
			 * managed by atomic_ops(3).
			 */
			*resultp = f->f_...;
			error = 0;
			break;
		}
	}
	pserialize_read_exit(s);

	return error;

Remove a frotz, as a writer, and free it once there are no more readers:

	struct frotz **fp, *f;

	mutex_enter(&frobbotzim.lock);
	for (fp = &frobbotzim.first; (f = *fp) != NULL; fp = &f->f_next) {
		if (f->f_... == key) {
			/*
			 * Unhook it from the list.  Readers may still
			 * be traversing the list at this point, so
			 * the next pointer must remain valid and
			 * memory must remain allocated.
			 */
			*fp = f->f_next;
			break;
		}
	}
	/*
	 * Wait for all existing readers to complete.  New readers will
	 * not see f because the list no longer points to it.
	 */
	pserialize_perform(frobbotzim.psz);
	/* Now nobody else can be touching f, so it is safe to free.  */
	mutex_exit(&frobbotzim.lock);

	if (f != NULL)
		pool_put(&frotz_pool, f);

The pserialize is implemented within the file sys/kern/subr_pserialize.c.

membar_ops(3), condvar(9), mutex(9), rwlock(9)

Hennessy, et al., Passive serialization in a multitasking environment, US Patent and Trademark Office, US Patent 4809168, February 28, 1989.

Passive serialization mechanism first appeared in NetBSD 6.0.
January 26, 2016 NetBSD 9.0