Re: [PATCH tip/core/rcu] classic RCU locking and memory-barrier cleanups

[Manfred Spraul <manfred@colorfullife.com>]

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Hi Paul,

You are beating me: I've just finished a my implementation, it's attached.
It boots with qemu, rcu torture enabled, both single and 8-cpu.

Two problems are open:
- right now, I don't use rcu_qsctr_inc() at all.
- qlowmark is set to 0, any other value breaks synchronize_rcu().

And I must read your implementation....

--
    Manfred

[-- Attachment #2: rcuclassic.c --]
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/*
 * Read-Copy Update mechanism for mutual exclusion
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 * Copyright IBM Corporation, 2001
 *
 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
 *	    Manfred Spraul <manfred@colorfullife.com>
 *
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 * Papers:
 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 * 		Documentation/RCU
 *
 * Rewrite based on a global state machine
 * (C) Manfred Spraul <manfred@colorfullife.com>, 2008
 *
 */
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/rcupdate.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <asm/atomic.h>
#include <linux/bitops.h>
#include <linux/module.h>
#include <linux/completion.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/time.h>
#include <linux/proc_fs.h>

#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_key;
struct lockdep_map rcu_lock_map =
	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
EXPORT_SYMBOL_GPL(rcu_lock_map);
#endif

/* Definition for rcupdate control block. */
static struct rcu_global_state rcu_global_state_normal = {
	.lock = __SEQLOCK_UNLOCKED(&rcu_global_state_normal.lock),
	.state = RCU_STATE_DESTROY,
	.start_immediately = 0,
	.cpus = __RCU_CPUMASK_INIT(&rcu_global_state_normal.cpus)
};

static struct rcu_global_state rcu_global_state_bh = {
	.lock = __SEQLOCK_UNLOCKED(&rcu_global_state_bh.lock),
	.state = RCU_STATE_DESTROY,
	.start_immediately = 0,
	.cpus = __RCU_CPUMASK_INIT(&rcu_global_state_bh.cpus)
};

DEFINE_PER_CPU(struct rcu_cpu_state, rcu_cpudata_normal) = { 0L };
DEFINE_PER_CPU(struct rcu_cpu_state, rcu_cpudata_bh) = { 0L };
DEFINE_PER_CPU(struct rcu_cpu_dead, rcu_cpudata_dead) = { 0L };


/* FIXME: setting qlowmark to non-zero causes a hang.
 * probably someone waits for a rcu completion - but
 * the real rcu cycle is never started because qlowmark is not
 * reached. (e.g. synchronize_rcu()).
 * idea: replace with a timer based delay.
 */
int qlowmark = 0;

void rcu_cpumask_init(struct rcu_cpumask *rcm)
{
	BUG_ON(!irqs_disabled());
	spin_lock(&rcm->lock);
	/*
	 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
	 * Barrier  Otherwise it can cause tickless idle CPUs to be
	 * included in rcp->cpumask, which will extend graceperiods
	 * unnecessarily.
	 */
	smp_mb();
	cpus_andnot(rcm->cpus, cpu_online_map, nohz_cpu_mask);

	spin_unlock(&rcm->lock);
}

int rcu_cpumask_clear_and_test(struct rcu_cpumask *rcm, int cpu)
{
	int ret = 0;

	BUG_ON(!irqs_disabled());
	spin_lock(&rcm->lock);
	cpu_clear(cpu, rcm->cpus);
	if (cpus_empty(rcm->cpus))
		ret = 1;
	spin_unlock(&rcm->lock);

	return ret;
}

long rcu_batches_completed(void)
{
	return rcu_global_state_normal.completed;
}

long rcu_batches_completed_bh(void)
{
	return rcu_global_state_normal.completed;
}

/**
 * rcu_state_startcycle - start the next rcu cycle
 * @rgs: global rcu state
 *
 * The function starts the next rcu cycle, either immediately or
 * by setting rgs->start_immediately.
 */ 
static void rcu_state_startcycle(struct rcu_global_state *rgs)
{
	unsigned seq;
	int do_real_start;

	BUG_ON(!irqs_disabled());
	do {
		seq = read_seqbegin(&rgs->lock);
		if (rgs->start_immediately == 0) {
			do_real_start = 1;
		} else {
			do_real_start = 0;
			BUG_ON(rgs->state == RCU_STATE_DESTROY);
		}
	} while (read_seqretry(&rgs->lock, seq));

	if (do_real_start) {
		write_seqlock(&rgs->lock);
		switch(rgs->state) {
		case RCU_STATE_DESTROY_AND_COLLECT:
		case RCU_STATE_GRACE:
			rgs->start_immediately = 1;
			break;
		case RCU_STATE_DESTROY:
			rgs->state = RCU_STATE_DESTROY_AND_COLLECT;
			BUG_ON(rgs->start_immediately);
			rcu_cpumask_init(&rgs->cpus);
			break;
		default:
			BUG();
		}
		write_sequnlock(&rgs->lock);
	}
}

static void rcu_checkqlen(struct rcu_global_state *rgs, struct rcu_cpu_state *rcs, int inc)
{
	BUG_ON(!irqs_disabled());
	rcs->newqlen += inc; 
	if (unlikely(rcs->newqlen > qlowmark)) {

		/* FIXME: actually, this code only needs to run once,
		 *  i.e. when qlen == qlowmark. But: qlowmark can be changed at runtime.
		 * and: doesn't work anyway, see comment near qlowmark
		 */
		rcu_state_startcycle(rgs);
	}
}


static void __call_rcu(struct rcu_head *head, struct rcu_global_state *rgs,
		struct rcu_cpu_state *rcs)
{
	if (rcs->new == NULL)
		rcs->newtail = &head->next;
	head->next = rcs->new;
	rcs->new = head;

	rcu_checkqlen(rgs, rcs, 1);
}

/**
 * call_rcu - Queue an RCU callback for invocation after a grace period.
 * @head: structure to be used for queueing the RCU updates.
 * @func: actual update function to be invoked after the grace period
 *
 * The update function will be invoked some time after a full grace
 * period elapses, in other words after all currently executing RCU
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 * and may be nested.
 */
void call_rcu(struct rcu_head *head,
				void (*func)(struct rcu_head *rcu))
{
	unsigned long flags;

	head->func = func;
	local_irq_save(flags);
	__call_rcu(head, &rcu_global_state_normal, &__get_cpu_var(rcu_cpudata_normal));
	local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu);

/**
 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 * @head: structure to be used for queueing the RCU updates.
 * @func: actual update function to be invoked after the grace period
 *
 * The update function will be invoked some time after a full grace
 * period elapses, in other words after all currently executing RCU
 * read-side critical sections have completed. call_rcu_bh() assumes
 * that the read-side critical sections end on completion of a softirq
 * handler. This means that read-side critical sections in process
 * context must not be interrupted by softirqs. This interface is to be
 * used when most of the read-side critical sections are in softirq context.
 * RCU read-side critical sections are delimited by rcu_read_lock() and
 * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
 * and rcu_read_unlock_bh(), if in process context. These may be nested.
 */
void call_rcu_bh(struct rcu_head *head,
				void (*func)(struct rcu_head *rcu))
{
	unsigned long flags;

	head->func = func;
	local_irq_save(flags);
	__call_rcu(head, &rcu_global_state_bh, &__get_cpu_var(rcu_cpudata_bh));
	local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu_bh);

#ifdef CONFIG_HOTPLUG_CPU

/**
 * rcu_bulk_add - bulk add new rcu objects.
 * @rgs: global rcu state
 * @rcs: cpu state
 * @h: linked list of rcu objects.
 *
 * Must be called with enabled local interrupts
 */
static void rcu_bulk_add(struct rcu_global_state *rgs, struct rcu_cpu_state *rcs, struct rcu_head *h, struct rcu_head **htail, int len)
{

	BUG_ON(irqs_disabled());

	if (len > 0) {
		local_irq_disable();
		if (rcs->new) {
			(*htail) = rcs->new;
			rcs->new = h;
		} else {
			rcs->new = h;
			rcs->newtail = htail;
		}
		rcu_checkqlen(rgs, rcs, len);
		local_irq_enable();
	}
}

#define RCU_BATCH_MIN		100
#define	RCU_BATCH_INCFACTOR	2
#define RCU_BATCH_DECFACTOR	4

static void rcu_move_and_raise(struct rcu_cpu_state *rcs)
{
	struct rcu_cpu_dead *rcd = &per_cpu(rcu_cpudata_dead, smp_processor_id());

	BUG_ON(!irqs_disabled());

	/* update batch limit:
	 * - if there are still old entries when new entries are added:
	 *   double the batch count.
	 * - if there are no old entries: reduce it by 25%, but never below 100.
	 */
	if (rcd->deadqlen)
		rcd->batchcount = rcd->batchcount*RCU_BATCH_INCFACTOR;
	 else
		rcd->batchcount = rcd->batchcount-rcd->batchcount/RCU_BATCH_DECFACTOR;
	if (rcd->batchcount < RCU_BATCH_MIN)
		rcd->batchcount = RCU_BATCH_MIN;

	if (rcs->oldqlen) {
		(*rcs->oldtail) = rcd->dead;
		rcd->dead = rcs->old;
		rcd->deadqlen += rcs->oldqlen;
		rcs->old = NULL;
		rcs->oldtail = NULL;
		rcs->oldqlen = 0;
	} 
	BUG_ON(rcs->old);
	BUG_ON(rcs->oldtail);
	BUG_ON(rcs->oldqlen);
	raise_softirq(RCU_SOFTIRQ);
}

static void rcu_state_machine(struct rcu_global_state *rgs, struct rcu_cpu_state *rcs, int is_quiet)
{
	int inc_state;
	unsigned seq;
	unsigned long flags;

	inc_state = 0;
	do {
		seq = read_seqbegin(&rgs->lock);
		local_irq_save(flags);
		if (rgs->state != rcs->state) {
			inc_state = 0;
			switch(rgs->state) {
			case RCU_STATE_DESTROY:
				rcs->state = rgs->state;
				rcu_move_and_raise(rcs);
				break;
			case RCU_STATE_DESTROY_AND_COLLECT:
				rcs->state = rgs->state;
				rcu_move_and_raise(rcs);
				rcs->old = rcs->new;
				rcs->oldtail = rcs->newtail;
				rcs->oldqlen = rcs->newqlen;
				rcs->new = NULL;
				rcs->newtail = NULL;
				rcs->newqlen = 0;
				if (rcu_cpumask_clear_and_test(&rgs->cpus, smp_processor_id()))
					inc_state = 1;
				break;
			case RCU_STATE_GRACE: 
				if (is_quiet) {
					rcs->state = rgs->state;
					if (rcu_cpumask_clear_and_test(&rgs->cpus, smp_processor_id()))
						inc_state = 1;
				}
				break;
			default:
				BUG();
			}
		}
		local_irq_restore(flags);
	} while (read_seqretry(&rgs->lock, seq));

	
	if (unlikely(inc_state)) {
		local_irq_save(flags);
		write_seqlock(&rgs->lock);
		/*
		 * double check for races: If e.g. a new cpu starts up it
		 * will call the state machine although it's not listed in the
		 * cpumasks. Then multiple cpu could could see the cleared bitmask
		 * and try to advance the state. In this case, only the first
		 * cpu does something, the remaining incs are ignored.
		 */
		if (rgs->state == rcs->state) {
			/*
			 * advance the state machine:
			 * - from COLLECT to GRACE
			 * - from GRACE to DESTROY/COLLECT
			 */
			switch(rgs->state) {
			case RCU_STATE_DESTROY_AND_COLLECT:
				rgs->state = RCU_STATE_GRACE;
				rcu_cpumask_init(&rgs->cpus);
				break;
			case RCU_STATE_GRACE:
				rgs->completed++;
				if (rgs->start_immediately) {
					rgs->state = RCU_STATE_DESTROY_AND_COLLECT;
					rcu_cpumask_init(&rgs->cpus);
				} else {
					rgs->state = RCU_STATE_DESTROY;
				}
				rgs->start_immediately = 0;
				break;
			default:
				BUG();
			}
		}
		write_sequnlock(&rgs->lock);
		local_irq_restore(flags);
	}
}

static void __rcu_offline_cpu(struct rcu_global_state *rgs, struct rcu_cpu_state *this_rcs,
					struct rcu_cpu_state *other_rcs, int cpu)
{
	/* task 1: move all entries from the new cpu into the lists of the current cpu.
	 * locking: The other cpu is dead, thus no locks are required.
	 *  Thus it's more or less a bulk call_rcu().
	 * For the sake of simplicity, all objects are treated as "new", even the objects
	 * that are already in old.
	 */
	rcu_bulk_add(rgs, this_rcs, other_rcs->new, other_rcs->newtail, other_rcs->newqlen);
	rcu_bulk_add(rgs, this_rcs, other_rcs->old, other_rcs->oldtail, other_rcs->oldqlen);


	/* task 2: handle the cpu bitmask of the other cpu
	 * We know that the other cpu is dead, thus it's guaranteed not to be holding
	 * any pointers to rcu protected objects.
	 */

	rcu_state_machine(rgs, other_rcs, 1);
}

static void rcu_offline_cpu(int cpu)
{
	struct rcu_cpu_state *this_rcs_normal = &get_cpu_var(rcu_cpudata_normal);
	struct rcu_cpu_state *this_rcs_bh = &get_cpu_var(rcu_cpudata_bh);

	BUG_ON(irqs_disabled());

	__rcu_offline_cpu(&rcu_global_state_normal, this_rcs_normal,
					&per_cpu(rcu_cpudata_normal, cpu), cpu);
	__rcu_offline_cpu(&rcu_global_state_bh, this_rcs_bh,
					&per_cpu(rcu_cpudata_bh, cpu), cpu);
	put_cpu_var(rcu_cpudata_normal);
	put_cpu_var(rcu_cpudata_bh);

	BUG_ON(rcu_needs_cpu(cpu));
}

#else

static void rcu_offline_cpu(int cpu)
{
}

#endif

static int __rcu_pending(struct rcu_global_state *rgs, struct rcu_cpu_state *rcs)
{
	/* quick and dirty check for pending */
	if (rgs->state != rcs->state)
		return 1;
	return 0;
}

/*
 * Check to see if there is any immediate RCU-related work to be done
 * by the current CPU, returning 1 if so.  This function is part of the
 * RCU implementation; it is -not- an exported member of the RCU API.
 */
int rcu_pending(int cpu)
{
	return __rcu_pending(&rcu_global_state_normal, &per_cpu(rcu_cpudata_normal, cpu)) ||
		__rcu_pending(&rcu_global_state_bh, &per_cpu(rcu_cpudata_bh, cpu));
}

/*
 * Check to see if any future RCU-related work will need to be done
 * by the current CPU, even if none need be done immediately, returning
 * 1 if so.  This function is part of the RCU implementation; it is -not-
 * an exported member of the RCU API.
 */
int rcu_needs_cpu(int cpu)
{
	struct rcu_cpu_state *rcs_normal = &per_cpu(rcu_cpudata_normal, cpu);
	struct rcu_cpu_state *rcs_bh = &per_cpu(rcu_cpudata_bh, cpu);

	return !!rcs_normal->new || !!rcs_normal->old ||
		!!rcs_bh->new || !!rcs_bh->old ||
		rcu_pending(cpu);
}

/**
 * rcu_check_callback(cpu, user) - external entry point for grace checking
 * @cpu: cpu id.
 * @user: user space was interrupted.
 *
 * Top-level function driving RCU grace-period detection, normally
 * invoked from the scheduler-clock interrupt.  This function simply
 * increments counters that are read only from softirq by this same
 * CPU, so there are no memory barriers required.
 *
 * This function can run with disabled local interrupts, thus all
 * callees must use local_irq_save()
 */
void rcu_check_callbacks(int cpu, int user)
{
	if (user ||
	    (idle_cpu(cpu) && !in_softirq() &&
				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {

		/*
		 * Get here if this CPU took its interrupt from user
		 * mode or from the idle loop, and if this is not a
		 * nested interrupt.  In this case, the CPU is in
		 * a quiescent state, so count it.
		 *
		 */
		rcu_state_machine(&rcu_global_state_normal, &per_cpu(rcu_cpudata_normal, cpu), 1);
		rcu_state_machine(&rcu_global_state_bh, &per_cpu(rcu_cpudata_bh, cpu), 1);

	} else if (!in_softirq()) {

		/*
		 * Get here if this CPU did not take its interrupt from
		 * softirq, in other words, if it is not interrupting
		 * a rcu_bh read-side critical section.  This is an _bh
		 * critical section, so count it.
		 */
		rcu_state_machine(&rcu_global_state_normal, &per_cpu(rcu_cpudata_normal, cpu), 0);
		rcu_state_machine(&rcu_global_state_bh, &per_cpu(rcu_cpudata_bh, cpu), 1);
	} else {
		/*
		 * We are interrupting something. Nevertheless - check if we should collect
		 * rcu objects. This can be done from arbitrary context.
		 */
		rcu_state_machine(&rcu_global_state_normal, &per_cpu(rcu_cpudata_normal, cpu), 0);
		rcu_state_machine(&rcu_global_state_bh, &per_cpu(rcu_cpudata_bh, cpu), 0);
	}
}

void rcu_restart_cpu(int cpu)
{
	BUG_ON(per_cpu(rcu_cpudata_normal, cpu).new != NULL);
	BUG_ON(per_cpu(rcu_cpudata_normal, cpu).old != NULL);
	per_cpu(rcu_cpudata_normal, cpu).state = RCU_STATE_DESTROY;

	BUG_ON(per_cpu(rcu_cpudata_bh, cpu).new != NULL);
	BUG_ON(per_cpu(rcu_cpudata_bh, cpu).old != NULL);
	per_cpu(rcu_cpudata_bh, cpu).state = RCU_STATE_DESTROY;
}

/*
 * Invoke the completed RCU callbacks.
 */
static void rcu_do_batch(struct rcu_cpu_dead *rcd)
{
	struct rcu_head *list;
	int i, count;

	if (!rcd->deadqlen)
		return;

	/* step 1: pull up to rcs->batchcount objects */
	BUG_ON(irqs_disabled());
	local_irq_disable();

	if (rcd->deadqlen > rcd->batchcount) {
		struct rcu_head *walk;

		list = rcd->dead;
		count = rcd->batchcount;
		
		walk = rcd->dead;
		for (i=0;i<count;i++)
			walk = walk->next;		
		rcd->dead = walk;

	} else {
		list = rcd->dead;
		count = rcd->deadqlen;

		rcd->dead = NULL;
	}
	rcd->deadqlen -= count;
	BUG_ON(rcd->deadqlen < 0);

	local_irq_enable();

	/* step 2: call the rcu callbacks */

	for (i=0;i<count;i++) {
		struct rcu_head *next;

		next = list->next;
		prefetch(next);
		list->func(list);
		list = next;
	}

	/* step 3: if still entries left, raise the softirq again */
	if (rcd->deadqlen)
		raise_softirq(RCU_SOFTIRQ);
}

static void rcu_process_callbacks(struct softirq_action *unused)
{
	rcu_do_batch(&per_cpu(rcu_cpudata_dead, smp_processor_id()));
}

static void rcu_init_percpu_data(struct rcu_global_state *rgs, struct rcu_cpu_state *rcs)
{
	rcs->new = rcs->old = NULL;
	rcs->newqlen = rcs->oldqlen = 0;
	rcs->state = RCU_STATE_DESTROY;
}

static void __cpuinit rcu_online_cpu(int cpu)
{
	rcu_init_percpu_data(&rcu_global_state_normal, &per_cpu(rcu_cpudata_normal, cpu));
	rcu_init_percpu_data(&rcu_global_state_bh, &per_cpu(rcu_cpudata_bh, cpu));

	per_cpu(rcu_cpudata_dead, cpu).dead = NULL;
	per_cpu(rcu_cpudata_dead, cpu).deadqlen = 0;
	per_cpu(rcu_cpudata_dead, cpu).batchcount = RCU_BATCH_MIN;

	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
}

static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
				unsigned long action, void *hcpu)
{
	long cpu = (long)hcpu;

	switch (action) {
	case CPU_UP_PREPARE:
	case CPU_UP_PREPARE_FROZEN:
		rcu_online_cpu(cpu);
		break;
	case CPU_DEAD:
	case CPU_DEAD_FROZEN:
		rcu_offline_cpu(cpu);
		break;
	default:
		break;
	}
	return NOTIFY_OK;
}

static struct notifier_block __cpuinitdata rcu_nb = {
	.notifier_call	= rcu_cpu_notify,
};

/*
 * Initializes rcu mechanism.  Assumed to be called early.
 * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
 * Note that rcu_qsctr and friends are implicitly
 * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
 */
void __init __rcu_init(void)
{
	rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
			(void *)(long)smp_processor_id());
	/* Register notifier for non-boot CPUs */
	register_cpu_notifier(&rcu_nb);
}

module_param(qlowmark, int, 0);



[-- Attachment #3: rcuclassic.h --]
[-- Type: text/plain, Size: 5396 bytes --]

/*
 * Read-Copy Update mechanism for mutual exclusion (classic version)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 * Copyright IBM Corporation, 2001
 *
 * Author: Dipankar Sarma <dipankar@in.ibm.com>
 *
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 * Papers:
 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 * 		Documentation/RCU
 *
 * Rewrite based on a global state machine
 * (C) Manfred Spraul <manfred@colorfullife.com>, 2008
 */

#ifndef __LINUX_RCUCLASSIC_H
#define __LINUX_RCUCLASSIC_H

#include <linux/cache.h>
#include <linux/spinlock.h>
#include <linux/threads.h>
#include <linux/percpu.h>
#include <linux/cpumask.h>
#include <linux/seqlock.h>
#include <linux/cpumask.h>

/*
 * cpu bitmask:
 * default implementation, flat without hierarchy, not optimized for UP.
 */

struct rcu_cpumask {
	spinlock_t lock;
	cpumask_t cpus;
} ____cacheline_internodealigned_in_smp;

#define __RCU_CPUMASK_INIT(ptr) { .lock = __SPIN_LOCK_UNLOCKED(&(ptr)->lock) }

/*
 * global state machine:
 * - each cpu regularly check the global state and compares it with it's own local state.
 * - if both state do not match, then the cpus do the required work and afterwards
 *   - update their local state
 *   - clear their bit in the cpu bitmask.
 * The state machine is sequence lock protected. It's only read with disabled local interupts.
 * Since all cpus must do something to complete a state change, the current state cannot
 * jump forward by more than one state.
 */

/* RCU_STATE_DESTROY:
 * call callbacks that were registered by call_rcu for the objects in rcu_cpu_state.old
 */
#define RCU_STATE_DESTROY		1
/* RCU_STATE_DESTROY_AND_COLLECT:
 * - call callbacks that were registered by call_rcu for the objects in rcu_cpu_state.old
 * - move the objects from rcu_cpu_state.new to rcu_cpu_state.new
 */
#define RCU_STATE_DESTROY_AND_COLLECT	2
/* RCU_STATE_GRACE
 * - wait for a quiescent state
 */
#define RCU_STATE_GRACE			3

struct rcu_global_state {
	seqlock_t		lock;
	int			state;
	int			start_immediately;
	long			completed;
	struct rcu_cpumask	cpus;
} ____cacheline_internodealigned_in_smp;

struct rcu_cpu_state {

	int state;

	/* new objects, directly from call_rcu().
	 * objects are added LIFO, better for cache hits.
	 * the list are length-based, not NULL-terminated.
	 */
	struct rcu_head *new;	/* new objects */
	struct rcu_head **newtail;
	long            newqlen; 	 /* # of queued callbacks */

	/* objects that are in rcu grace processing. The actual
	 * state depends on rgs->state.
	 */
	struct rcu_head *old;		
	struct rcu_head **oldtail;
	long            oldqlen;
};

struct rcu_cpu_dead {
	/* objects that are scheduled for immediate call of
	 * ->func().
	 * objects are added FIFO, necessary for forward progress.
	 * only one structure for _bh and _normal.
	 */
	struct rcu_head *dead;
	long		deadqlen;

	long		batchcount;
};

DECLARE_PER_CPU(struct rcu_cpu_state, rcu_cpudata_normal);
DECLARE_PER_CPU(struct rcu_cpu_state, rcu_cpudata_bh);
DECLARE_PER_CPU(struct rcu_cpu_dead, rcu_cpudata_dead);

extern long rcu_batches_completed(void);
extern long rcu_batches_completed_bh(void);

extern int rcu_pending(int cpu);
extern int rcu_needs_cpu(int cpu);

#ifdef CONFIG_DEBUG_LOCK_ALLOC
extern struct lockdep_map rcu_lock_map;
# define rcu_read_acquire()	\
			lock_acquire(&rcu_lock_map, 0, 0, 2, 1, _THIS_IP_)
# define rcu_read_release()	lock_release(&rcu_lock_map, 1, _THIS_IP_)
#else
# define rcu_read_acquire()	do { } while (0)
# define rcu_read_release()	do { } while (0)
#endif

#define __rcu_read_lock() \
	do { \
		preempt_disable(); \
		__acquire(RCU); \
		rcu_read_acquire(); \
	} while (0)
#define __rcu_read_unlock() \
	do { \
		rcu_read_release(); \
		__release(RCU); \
		preempt_enable(); \
	} while (0)
#define __rcu_read_lock_bh() \
	do { \
		local_bh_disable(); \
		__acquire(RCU_BH); \
		rcu_read_acquire(); \
	} while (0)
#define __rcu_read_unlock_bh() \
	do { \
		rcu_read_release(); \
		__release(RCU_BH); \
		local_bh_enable(); \
	} while (0)

#define __synchronize_sched() synchronize_rcu()

#define call_rcu_sched(head, func) call_rcu(head, func)

extern void __rcu_init(void);
#define rcu_init_sched()	do { } while (0)
extern void rcu_check_callbacks(int cpu, int user);
extern void rcu_restart_cpu(int cpu);


#define rcu_enter_nohz()	do { } while (0)
#define rcu_exit_nohz()		do { } while (0)

#define rcu_qsctr_inc(cpu)	do { } while (0)
#define rcu_bh_qsctr_inc(cpu)	do { } while (0)

#endif /* __LINUX_RCUCLASSIC_H */