xref: /src/sys/kern/kern_turnstile.c

/*	$NetBSD: kern_turnstile.c,v 1.1.36.2 2006/10/20 19:38:44 ad Exp $	*/

/*-
 * Copyright (c) 2002, 2006 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe and Andrew Doran.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the NetBSD
 *	Foundation, Inc. and its contributors.
 * 4. Neither the name of The NetBSD Foundation nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Turnstiles are described in detail in:
 *
 *	Solaris Internals: Core Kernel Architecture, Jim Mauro and
 *	    Richard McDougall.
 *
 * Turnstiles are kept in a hash table.  There are likely to be many more
 * synchronisation objects than there are threads.  Since a thread can block
 * on only one lock at a time, we only need one turnstile per thread, and
 * so they are allocated at thread creation time.
 *
 * When a thread decides it needs to block on a lock, it looks up the
 * active turnstile for that lock.  If no active turnstile exists, then
 * the process lends its turnstile to the lock.  If there is already an
 * active turnstile for the lock, the thread places its turnstile on a
 * list of free turnstiles, and references the active one instead.
 *
 * The act of looking up the turnstile acquires an interlock on the sleep
 * queue.  If a thread decides it doesn't need to block after all, then this
 * interlock must be released by explicitly aborting the turnstile
 * operation.
 *
 * When a thread is awakened, it needs to get its turnstile back.  If there
 * are still other threads waiting in the active turnstile, the the thread
 * grabs a free turnstile off the free list.  Otherwise, it can take back
 * the active turnstile from the lock (thus deactivating the turnstile).
 *
 * Turnstiles are the place to do priority inheritence.  However, we do
 * not currently implement that.
 */

#include "opt_lockdebug.h"
#include "opt_multiprocessor.h"

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_turnstile.c,v 1.1.36.2 2006/10/20 19:38:44 ad Exp $");

#include <sys/param.h>
#include <sys/lock.h>
#include <sys/pool.h>
#include <sys/proc.h>
#include <sys/sleepq.h>
#include <sys/systm.h>

#define	TS_HASH_SIZE	64
#define	TS_HASH_MASK	(TS_HASH_SIZE - 1)
#define	TS_HASH(obj)	(((uintptr_t)(obj) >> 8) & TS_HASH_MASK)

tschain_t	turnstile_tab[TS_HASH_SIZE];

struct pool turnstile_pool;
struct pool_cache turnstile_cache;

int	turnstile_ctor(void *, void *, int);

extern turnstile_t turnstile0;

/*
 * turnstile_init:
 *
 *	Initialize the turnstile mechanism.
 */
void
turnstile_init(void)
{
	tschain_t *tc;
	int i;

	for (i = 0; i < TS_HASH_SIZE; i++) {
		tc = &turnstile_tab[i];
		mutex_init(&tc->tc_mutex, MUTEX_SPIN, IPL_SCHED);
		LIST_INIT(&tc->tc_chain);
	}

	pool_init(&turnstile_pool, sizeof(turnstile_t), 0, 0, 0,
	    "tspool", &pool_allocator_nointr);
	pool_cache_init(&turnstile_cache, &turnstile_pool,
	    turnstile_ctor, NULL, NULL);

	(void)turnstile_ctor(NULL, &turnstile0, 0);
}

/*
 * turnstile_ctor:
 *
 *	Constructor for turnstiles.
 */
int
turnstile_ctor(void *arg, void *obj, int flags)
{
	turnstile_t *ts = obj;

	memset(ts, 0, sizeof(*ts));
	sleepq_init(&ts->ts_sleepq[TS_READER_Q], NULL);
	sleepq_init(&ts->ts_sleepq[TS_WRITER_Q], NULL);
	return (0);
}

/*
 * turnstile_remove:
 *
 *	Remove an LWP from a turnstile sleep queue and wake it.
 */
static inline int
turnstile_remove(turnstile_t *ts, struct lwp *l, sleepq_t *sq)
{
	turnstile_t *nts;

	KASSERT(l->l_ts == ts);

	/*
	 * This process is no longer using the active turnstile.
	 * Find an inactive one on the free list to give to it.
	 */
	if ((nts = ts->ts_free) != NULL) {
		KASSERT(TS_ALL_WAITERS(ts) > 1);
		l->l_ts = nts;
		ts->ts_free = nts->ts_free;
		nts->ts_free = NULL;
	} else {
		/*
		 * If the free list is empty, this is the last
		 * waiter.
		 */
		KASSERT(TS_ALL_WAITERS(ts) == 1);
		LIST_REMOVE(ts, ts_chain);
	}

	return sleepq_remove(sq, l);
}

/*
 * turnstile_lookup:
 *
 *	Look up the turnstile for the specified lock.  This acquires and
 *	holds the turnstile chain lock (sleep queue interlock).
 */
turnstile_t *
turnstile_lookup(wchan_t obj)
{
	turnstile_t *ts;
	tschain_t *tc;

	tc = &turnstile_tab[TS_HASH(obj)];

	mutex_enter(&tc->tc_mutex);

	LIST_FOREACH(ts, &tc->tc_chain, ts_chain)
		if (ts->ts_obj == obj)
			return (ts);

	/*
	 * No turnstile yet for this lock.  No problem, turnstile_block()
	 * handles this by fetching the turnstile from the blocking thread.
	 */
	return (NULL);
}

/*
 * turnstile_exit:
 *
 *	Abort a turnstile operation.
 */
void
turnstile_exit(wchan_t obj)
{
	tschain_t *tc;

	tc = &turnstile_tab[TS_HASH(obj)];
	mutex_exit(&tc->tc_mutex);
}

/*
 * turnstile_block:
 *
 *	 Enter an object into the turnstile chain and prepare the current
 *	 LWP for sleep.
 */
void
turnstile_block(turnstile_t *ts, int q, int pri, wchan_t obj)
{
	struct lwp *l;
	turnstile_t *ots;
	tschain_t *tc;
	sleepq_t *sq;

	KASSERT(q == TS_READER_Q || q == TS_WRITER_Q);

	tc = &turnstile_tab[TS_HASH(obj)];
	l = curlwp;

	LOCK_ASSERT(mutex_owned(&tc->tc_mutex));
	KASSERT(l != NULL && l->l_ts != NULL);

	if (ts == NULL) {
		/*
		 * We are the first thread to wait for this object;
		 * lend our turnstile to it.
		 */
		ts = l->l_ts;
		KASSERT(TS_ALL_WAITERS(ts) == 0);
		KASSERT(TAILQ_EMPTY(&ts->ts_sleepq[TS_READER_Q].sq_queue) &&
			TAILQ_EMPTY(&ts->ts_sleepq[TS_WRITER_Q].sq_queue));
		ts->ts_obj = obj;
		LIST_INSERT_HEAD(&tc->tc_chain, ts, ts_chain);
	} else {
		/*
		 * Object already has a turnstile.  Put our turnstile
		 * onto the free list, and reference the existing
		 * turnstile instead.
		 */
		ots = l->l_ts;
		ots->ts_free = ts->ts_free;
		ts->ts_free = ots;
		l->l_ts = ts;

		KASSERT(TS_ALL_WAITERS(ts) != 0);
		KASSERT(!TAILQ_EMPTY(&ts->ts_sleepq[TS_READER_Q].sq_queue) ||
			!TAILQ_EMPTY(&ts->ts_sleepq[TS_WRITER_Q].sq_queue));
	}

	sq = &ts->ts_sleepq[q];
	sq->sq_mutex = &tc->tc_mutex;
	sleepq_enter(sq, pri, obj, "tstile", 0, 0);
	(void)sleepq_block(sq, 0);
}

/*
 * turnstile_wakeup:
 *
 *	Wake up the specified number of threads that are blocked
 *	in a turnstile.
 */
void
turnstile_wakeup(turnstile_t *ts, int rw, int count, struct lwp *nl)
{
	sleepq_t *sq;
	tschain_t *tc;
	struct lwp *l;
	int swapin;

	KASSERT(rw == TS_READER_Q || rw == TS_WRITER_Q);
	KASSERT(count > 0);

	swapin = 0;
	tc = &turnstile_tab[TS_HASH(ts->ts_obj)];
	sq = &ts->ts_sleepq[rw];

	LOCK_ASSERT(mutex_owned(&tc->tc_mutex) && sq->sq_mutex == &tc->tc_mutex);

	mutex_enter(&sched_mutex);
	if (nl != NULL) {
#if defined(DEBUG) || defined(LOCKDEBUG)
		TAILQ_FOREACH(l, &sq->sq_queue, l_sleepq) {
			if (l == nl)
				break;
		}
		if (l == NULL)
			panic("turnstile_wakeup: nl not on sleepq");
#endif
		swapin |= turnstile_remove(ts, nl, sq);
	} else {
		while (count-- > 0) {
			l = TAILQ_FIRST(&sq->sq_queue);
			KASSERT(l != NULL);
			swapin |= turnstile_remove(ts, l, sq);
		}
	}
	mutex_exit(&sched_mutex);
	mutex_exit(&tc->tc_mutex);

	/*
	 * If there are newly awakend threads that need to be swapped in,
	 * then kick the swapper into action.
	 */
	if (swapin)
		wakeup(&proc0);
}