Index: task.c
===================================================================
RCS file: task.c
diff -N task.c
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ task.c	13 Sep 2017 00:05:10 -0000
@@ -0,0 +1,156 @@
+/*	$OpenBSD$ */
+
+/*
+ * Copyright (c) 2017 David Gwynne <dlg@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <errno.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <pthread_np.h>
+
+#include "task.h"
+
+#define ISSET(_v, _m)	((_v) & (_m))
+#define SET(_v, _m)	((_v) |= (_m))
+#define CLR(_v, _m)	((_v) &= ~(_m))
+
+struct taskq {
+	pthread_t		  thread;
+	struct task_list	  list;
+	pthread_mutex_t		  mtx;
+	pthread_cond_t		  cv;
+};
+
+#define TASK_ONQUEUE		(1 << 0)
+
+static void *taskq_run(void *);
+
+struct taskq *
+taskq_create(const char *name)
+{
+	struct taskq *tq;
+	int error;
+
+	tq = malloc(sizeof(*tq));
+	if (tq == NULL)
+		return (NULL);
+
+	TAILQ_INIT(&tq->list);
+
+	error = pthread_mutex_init(&tq->mtx, NULL);
+	if (error != 0)
+		goto free;
+
+	error = pthread_cond_init(&tq->cv, NULL);
+	if (error != 0)
+		goto mtx;
+
+	error = pthread_create(&tq->thread, NULL, taskq_run, tq);
+	if (error != 0)
+		goto cv;
+
+	pthread_set_name_np(tq->thread, name);
+
+	return (tq);
+
+cv:
+	pthread_cond_destroy(&tq->cv);
+mtx:
+	pthread_mutex_destroy(&tq->mtx); /* can this really fail? */
+free:
+	free(tq);
+
+	errno = error;
+	return (NULL);
+}
+
+static void *
+taskq_run(void *tqarg)
+{
+	struct taskq *tq = tqarg;
+	struct task *t;
+
+	void (*t_func)(void *);
+	void *t_arg;
+
+	for (;;) {
+		pthread_mutex_lock(&tq->mtx);
+		while ((t = TAILQ_FIRST(&tq->list)) == NULL)
+			pthread_cond_wait(&tq->cv, &tq->mtx);
+
+		TAILQ_REMOVE(&tq->list, t, t_entry);
+		CLR(t->t_flags, TASK_ONQUEUE);
+
+		t_func = t->t_func;
+		t_arg = t->t_arg;
+
+		pthread_mutex_unlock(&tq->mtx);
+
+		(*t_func)(t_arg);
+	}
+
+	return (NULL);
+}
+
+void
+task_set(struct task *t, void (*fn)(void *), void *arg)
+{
+	t->t_func = fn;
+	t->t_arg = arg;
+	t->t_flags = 0;
+}
+
+int
+task_add(struct taskq *tq, struct task *t)
+{
+	int rv = 1;
+
+	if (ISSET(t->t_flags, TASK_ONQUEUE))
+		return (0);
+
+	pthread_mutex_lock(&tq->mtx);
+	if (ISSET(t->t_flags, TASK_ONQUEUE))
+		rv = 0;
+	else {
+		SET(t->t_flags, TASK_ONQUEUE);
+		TAILQ_INSERT_TAIL(&tq->list, t, t_entry);
+		pthread_cond_signal(&tq->cv);
+	}
+	pthread_mutex_unlock(&tq->mtx);
+
+	return (rv);
+}
+
+int
+task_del(struct taskq *tq, struct task *t)
+{
+	int rv = 1;
+
+	if (!ISSET(t->t_flags, TASK_ONQUEUE))
+		return (0);
+
+	pthread_mutex_lock(&tq->mtx);
+	if (!ISSET(t->t_flags, TASK_ONQUEUE))
+		rv = 0;
+	else {
+		TAILQ_REMOVE(&tq->list, t, t_entry);
+		CLR(t->t_flags, TASK_ONQUEUE);
+	}
+	pthread_mutex_unlock(&tq->mtx);
+
+	return (rv);
+} 
Index: task.h
===================================================================
RCS file: task.h
diff -N task.h
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ task.h	13 Sep 2017 00:05:10 -0000
@@ -0,0 +1,43 @@
+/*	$OpenBSD: task.h,v 1.11 2016/06/07 07:53:33 mpi Exp $ */
+
+/*
+ * Copyright (c) 2013 David Gwynne <dlg@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef _TASK_H_
+#define _TASK_H_
+
+#include <sys/queue.h>
+
+struct taskq;
+
+struct task {
+	TAILQ_ENTRY(task) t_entry;
+	void		(*t_func)(void *);
+	void		*t_arg;
+	unsigned int	t_flags;
+};
+
+TAILQ_HEAD(task_list, task);
+
+#define TASK_INITIALIZER(_f, _a)  {{ NULL, NULL }, (_f), (_a), 0 }
+
+struct taskq	*taskq_create(const char *);
+
+void		 task_set(struct task *, void (*)(void *), void *);
+int		 task_add(struct taskq *, struct task *);
+int		 task_del(struct taskq *, struct task *);
+
+#endif /* _TASK_H_ */