kernel/rcutree_plugin.h - kernel/prism - Git at Google

 /*
  * Read-Copy Update mechanism for mutual exclusion (tree-based version)
  * Internal non-public definitions that provide either classic
  * or preemptable semantics.
  *
  * 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 Red Hat, 2009
  * Copyright IBM Corporation, 2009
  *
  * Author: Ingo Molnar <mingo@elte.hu>
  *	   Paul E. McKenney <paulmck@linux.vnet.ibm.com>
  */


 #ifdef CONFIG_TREE_PREEMPT_RCU

 struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt_state);
 DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);

 /*
  * Tell them what RCU they are running.
  */
 static void rcu_bootup_announce(void)
 {
 	printk(KERN_INFO
 	       "Experimental preemptable hierarchical RCU implementation.\n");
 }

 /*
  * Return the number of RCU-preempt batches processed thus far
  * for debug and statistics.
  */
 long rcu_batches_completed_preempt(void)
 {
 	return rcu_preempt_state.completed;
 }
 EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);

 /*
  * Return the number of RCU batches processed thus far for debug & stats.
  */
 long rcu_batches_completed(void)
 {
 	return rcu_batches_completed_preempt();
 }
 EXPORT_SYMBOL_GPL(rcu_batches_completed);

 /*
  * Record a preemptable-RCU quiescent state for the specified CPU.  Note
  * that this just means that the task currently running on the CPU is
  * not in a quiescent state.  There might be any number of tasks blocked
  * while in an RCU read-side critical section.
  */
 static void rcu_preempt_qs(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
 	rdp->passed_quiesc_completed = rdp->completed;
 	barrier();
 	rdp->passed_quiesc = 1;
 }

 /*
  * We have entered the scheduler, and the current task might soon be
  * context-switched away from.  If this task is in an RCU read-side
  * critical section, we will no longer be able to rely on the CPU to
  * record that fact, so we enqueue the task on the appropriate entry
  * of the blocked_tasks[] array.  The task will dequeue itself when
  * it exits the outermost enclosing RCU read-side critical section.
  * Therefore, the current grace period cannot be permitted to complete
  * until the blocked_tasks[] entry indexed by the low-order bit of
  * rnp->gpnum empties.
  *
  * Caller must disable preemption.
  */
 static void rcu_preempt_note_context_switch(int cpu)
 {
 	struct task_struct *t = current;
 	unsigned long flags;
 	int phase;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;

 	if (t->rcu_read_lock_nesting &&
 	    (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {

 		/* Possibly blocking in an RCU read-side critical section. */
 		rdp = rcu_preempt_state.rda[cpu];
 		rnp = rdp->mynode;
 		spin_lock_irqsave(&rnp->lock, flags);
 		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
 		t->rcu_blocked_node = rnp;

 		/*
 		 * If this CPU has already checked in, then this task
 		 * will hold up the next grace period rather than the
 		 * current grace period.  Queue the task accordingly.
 		 * If the task is queued for the current grace period
 		 * (i.e., this CPU has not yet passed through a quiescent
 		 * state for the current grace period), then as long
 		 * as that task remains queued, the current grace period
 		 * cannot end.
 		 *
 		 * But first, note that the current CPU must still be
 		 * on line!
 		 */
 		WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
 		WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
 		phase = (rnp->gpnum + !(rnp->qsmask & rdp->grpmask)) & 0x1;
 		list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
 		spin_unlock_irqrestore(&rnp->lock, flags);
 	}

 	/*
 	 * Either we were not in an RCU read-side critical section to
 	 * begin with, or we have now recorded that critical section
 	 * globally.  Either way, we can now note a quiescent state
 	 * for this CPU.  Again, if we were in an RCU read-side critical
 	 * section, and if that critical section was blocking the current
 	 * grace period, then the fact that the task has been enqueued
 	 * means that we continue to block the current grace period.
 	 */
 	rcu_preempt_qs(cpu);
 	local_irq_save(flags);
 	t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 	local_irq_restore(flags);
 }

 /*
  * Tree-preemptable RCU implementation for rcu_read_lock().
  * Just increment ->rcu_read_lock_nesting, shared state will be updated
  * if we block.
  */
 void __rcu_read_lock(void)
 {
 	ACCESS_ONCE(current->rcu_read_lock_nesting)++;
 	barrier();  /* needed if we ever invoke rcu_read_lock in rcutree.c */
 }
 EXPORT_SYMBOL_GPL(__rcu_read_lock);

 /*
  * Check for preempted RCU readers blocking the current grace period
  * for the specified rcu_node structure.  If the caller needs a reliable
  * answer, it must hold the rcu_node's ->lock.
  */
 static int rcu_preempted_readers(struct rcu_node *rnp)
 {
 	return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
 }

 static void rcu_read_unlock_special(struct task_struct *t)
 {
 	int empty;
 	unsigned long flags;
 	unsigned long mask;
 	struct rcu_node *rnp;
 	int special;

 	/* NMI handlers cannot block and cannot safely manipulate state. */
 	if (in_nmi())
 		return;

 	local_irq_save(flags);

 	/*
 	 * If RCU core is waiting for this CPU to exit critical section,
 	 * let it know that we have done so.
 	 */
 	special = t->rcu_read_unlock_special;
 	if (special & RCU_READ_UNLOCK_NEED_QS) {
 		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 		rcu_preempt_qs(smp_processor_id());
 	}

 	/* Hardware IRQ handlers cannot block. */
 	if (in_irq()) {
 		local_irq_restore(flags);
 		return;
 	}

 	/* Clean up if blocked during RCU read-side critical section. */
 	if (special & RCU_READ_UNLOCK_BLOCKED) {
 		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;

 		/*
 		 * Remove this task from the list it blocked on.  The
 		 * task can migrate while we acquire the lock, but at
 		 * most one time.  So at most two passes through loop.
 		 */
 		for (;;) {
 			rnp = t->rcu_blocked_node;
 			spin_lock(&rnp->lock);  /* irqs already disabled. */
 			if (rnp == t->rcu_blocked_node)
 				break;
 			spin_unlock(&rnp->lock);  /* irqs remain disabled. */
 		}
 		empty = !rcu_preempted_readers(rnp);
 		list_del_init(&t->rcu_node_entry);
 		t->rcu_blocked_node = NULL;

 		/*
 		 * If this was the last task on the current list, and if
 		 * we aren't waiting on any CPUs, report the quiescent state.
 		 * Note that both cpu_quiet_msk_finish() and cpu_quiet_msk()
 		 * drop rnp->lock and restore irq.
 		 */
 		if (!empty && rnp->qsmask == 0 &&
 		    !rcu_preempted_readers(rnp)) {
 			struct rcu_node *rnp_p;

 			if (rnp->parent == NULL) {
 				/* Only one rcu_node in the tree. */
 				cpu_quiet_msk_finish(&rcu_preempt_state, flags);
 				return;
 			}
 			/* Report up the rest of the hierarchy. */
 			mask = rnp->grpmask;
 			spin_unlock_irqrestore(&rnp->lock, flags);
 			rnp_p = rnp->parent;
 			spin_lock_irqsave(&rnp_p->lock, flags);
 			WARN_ON_ONCE(rnp->qsmask);
 			cpu_quiet_msk(mask, &rcu_preempt_state, rnp_p, flags);
 			return;
 		}
 		spin_unlock(&rnp->lock);
 	}
 	local_irq_restore(flags);
 }

 /*
  * Tree-preemptable RCU implementation for rcu_read_unlock().
  * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
  * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
  * invoke rcu_read_unlock_special() to clean up after a context switch
  * in an RCU read-side critical section and other special cases.
  */
 void __rcu_read_unlock(void)
 {
 	struct task_struct *t = current;

 	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
 	if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
 	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
 		rcu_read_unlock_special(t);
 }
 EXPORT_SYMBOL_GPL(__rcu_read_unlock);

 #ifdef CONFIG_RCU_CPU_STALL_DETECTOR

 /*
  * Scan the current list of tasks blocked within RCU read-side critical
  * sections, printing out the tid of each.
  */
 static void rcu_print_task_stall(struct rcu_node *rnp)
 {
 	unsigned long flags;
 	struct list_head *lp;
 	int phase;
 	struct task_struct *t;

 	if (rcu_preempted_readers(rnp)) {
 		spin_lock_irqsave(&rnp->lock, flags);
 		phase = rnp->gpnum & 0x1;
 		lp = &rnp->blocked_tasks[phase];
 		list_for_each_entry(t, lp, rcu_node_entry)
 			printk(" P%d", t->pid);
 		spin_unlock_irqrestore(&rnp->lock, flags);
 	}
 }

 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

 /*
  * Check that the list of blocked tasks for the newly completed grace
  * period is in fact empty.  It is a serious bug to complete a grace
  * period that still has RCU readers blocked!  This function must be
  * invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
  * must be held by the caller.
  */
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
 	WARN_ON_ONCE(rcu_preempted_readers(rnp));
 	WARN_ON_ONCE(rnp->qsmask);
 }

 #ifdef CONFIG_HOTPLUG_CPU

 /*
  * Handle tasklist migration for case in which all CPUs covered by the
  * specified rcu_node have gone offline.  Move them up to the root
  * rcu_node.  The reason for not just moving them to the immediate
  * parent is to remove the need for rcu_read_unlock_special() to
  * make more than two attempts to acquire the target rcu_node's lock.
  *
  * Returns 1 if there was previously a task blocking the current grace
  * period on the specified rcu_node structure.
  *
  * The caller must hold rnp->lock with irqs disabled.
  */
 static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
 				     struct rcu_node *rnp,
 				     struct rcu_data *rdp)
 {
 	int i;
 	struct list_head *lp;
 	struct list_head *lp_root;
 	int retval = rcu_preempted_readers(rnp);
 	struct rcu_node *rnp_root = rcu_get_root(rsp);
 	struct task_struct *tp;

 	if (rnp == rnp_root) {
 		WARN_ONCE(1, "Last CPU thought to be offlined?");
 		return 0;  /* Shouldn't happen: at least one CPU online. */
 	}
 	WARN_ON_ONCE(rnp != rdp->mynode &&
 		     (!list_empty(&rnp->blocked_tasks[0]) ||
 		      !list_empty(&rnp->blocked_tasks[1])));

 	/*
 	 * Move tasks up to root rcu_node.  Rely on the fact that the
 	 * root rcu_node can be at most one ahead of the rest of the
 	 * rcu_nodes in terms of gp_num value.  This fact allows us to
 	 * move the blocked_tasks[] array directly, element by element.
 	 */
 	for (i = 0; i < 2; i++) {
 		lp = &rnp->blocked_tasks[i];
 		lp_root = &rnp_root->blocked_tasks[i];
 		while (!list_empty(lp)) {
 			tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
 			spin_lock(&rnp_root->lock); /* irqs already disabled */
 			list_del(&tp->rcu_node_entry);
 			tp->rcu_blocked_node = rnp_root;
 			list_add(&tp->rcu_node_entry, lp_root);
 			spin_unlock(&rnp_root->lock); /* irqs remain disabled */
 		}
 	}

 	return retval;
 }

 /*
  * Do CPU-offline processing for preemptable RCU.
  */
 static void rcu_preempt_offline_cpu(int cpu)
 {
 	__rcu_offline_cpu(cpu, &rcu_preempt_state);
 }

 #endif /* #ifdef CONFIG_HOTPLUG_CPU */

 /*
  * Check for a quiescent state from the current CPU.  When a task blocks,
  * the task is recorded in the corresponding CPU's rcu_node structure,
  * which is checked elsewhere.
  *
  * Caller must disable hard irqs.
  */
 static void rcu_preempt_check_callbacks(int cpu)
 {
 	struct task_struct *t = current;

 	if (t->rcu_read_lock_nesting == 0) {
 		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 		rcu_preempt_qs(cpu);
 		return;
 	}
 	if (per_cpu(rcu_preempt_data, cpu).qs_pending)
 		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
 }

 /*
  * Process callbacks for preemptable RCU.
  */
 static void rcu_preempt_process_callbacks(void)
 {
 	__rcu_process_callbacks(&rcu_preempt_state,
 				&__get_cpu_var(rcu_preempt_data));
 }

 /*
  * Queue a preemptable-RCU callback for invocation after a grace period.
  */
 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 {
 	__call_rcu(head, func, &rcu_preempt_state);
 }
 EXPORT_SYMBOL_GPL(call_rcu);

 /*
  * Wait for an rcu-preempt grace period.  We are supposed to expedite the
  * grace period, but this is the crude slow compatability hack, so just
  * invoke synchronize_rcu().
  */
 void synchronize_rcu_expedited(void)
 {
 	synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

 /*
  * Check to see if there is any immediate preemptable-RCU-related work
  * to be done.
  */
 static int rcu_preempt_pending(int cpu)
 {
 	return __rcu_pending(&rcu_preempt_state,
 			     &per_cpu(rcu_preempt_data, cpu));
 }

 /*
  * Does preemptable RCU need the CPU to stay out of dynticks mode?
  */
 static int rcu_preempt_needs_cpu(int cpu)
 {
 	return !!per_cpu(rcu_preempt_data, cpu).nxtlist;
 }

 /**
  * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
  */
 void rcu_barrier(void)
 {
 	_rcu_barrier(&rcu_preempt_state, call_rcu);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier);

 /*
  * Initialize preemptable RCU's per-CPU data.
  */
 static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
 {
 	rcu_init_percpu_data(cpu, &rcu_preempt_state, 1);
 }

 /*
  * Move preemptable RCU's callbacks to ->orphan_cbs_list.
  */
 static void rcu_preempt_send_cbs_to_orphanage(void)
 {
 	rcu_send_cbs_to_orphanage(&rcu_preempt_state);
 }

 /*
  * Initialize preemptable RCU's state structures.
  */
 static void __init __rcu_init_preempt(void)
 {
 	RCU_INIT_FLAVOR(&rcu_preempt_state, rcu_preempt_data);
 }

 /*
  * Check for a task exiting while in a preemptable-RCU read-side
  * critical section, clean up if so.  No need to issue warnings,
  * as debug_check_no_locks_held() already does this if lockdep
  * is enabled.
  */
 void exit_rcu(void)
 {
 	struct task_struct *t = current;

 	if (t->rcu_read_lock_nesting == 0)
 		return;
 	t->rcu_read_lock_nesting = 1;
 	rcu_read_unlock();
 }

 #else /* #ifdef CONFIG_TREE_PREEMPT_RCU */

 /*
  * Tell them what RCU they are running.
  */
 static void rcu_bootup_announce(void)
 {
 	printk(KERN_INFO "Hierarchical RCU implementation.\n");
 }

 /*
  * Return the number of RCU batches processed thus far for debug & stats.
  */
 long rcu_batches_completed(void)
 {
 	return rcu_batches_completed_sched();
 }
 EXPORT_SYMBOL_GPL(rcu_batches_completed);

 /*
  * Because preemptable RCU does not exist, we never have to check for
  * CPUs being in quiescent states.
  */
 static void rcu_preempt_note_context_switch(int cpu)
 {
 }

 /*
  * Because preemptable RCU does not exist, there are never any preempted
  * RCU readers.
  */
 static int rcu_preempted_readers(struct rcu_node *rnp)
 {
 	return 0;
 }

 #ifdef CONFIG_RCU_CPU_STALL_DETECTOR

 /*
  * Because preemptable RCU does not exist, we never have to check for
  * tasks blocked within RCU read-side critical sections.
  */
 static void rcu_print_task_stall(struct rcu_node *rnp)
 {
 }

 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

 /*
  * Because there is no preemptable RCU, there can be no readers blocked,
  * so there is no need to check for blocked tasks.  So check only for
  * bogus qsmask values.
  */
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
 	WARN_ON_ONCE(rnp->qsmask);
 }

 #ifdef CONFIG_HOTPLUG_CPU

 /*
  * Because preemptable RCU does not exist, it never needs to migrate
  * tasks that were blocked within RCU read-side critical sections, and
  * such non-existent tasks cannot possibly have been blocking the current
  * grace period.
  */
 static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
 				     struct rcu_node *rnp,
 				     struct rcu_data *rdp)
 {
 	return 0;
 }

 /*
  * Because preemptable RCU does not exist, it never needs CPU-offline
  * processing.
  */
 static void rcu_preempt_offline_cpu(int cpu)
 {
 }

 #endif /* #ifdef CONFIG_HOTPLUG_CPU */

 /*
  * Because preemptable RCU does not exist, it never has any callbacks
  * to check.
  */
 static void rcu_preempt_check_callbacks(int cpu)
 {
 }

 /*
  * Because preemptable RCU does not exist, it never has any callbacks
  * to process.
  */
 static void rcu_preempt_process_callbacks(void)
 {
 }

 /*
  * In classic RCU, call_rcu() is just call_rcu_sched().
  */
 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 {
 	call_rcu_sched(head, func);
 }
 EXPORT_SYMBOL_GPL(call_rcu);

 /*
  * Wait for an rcu-preempt grace period, but make it happen quickly.
  * But because preemptable RCU does not exist, map to rcu-sched.
  */
 void synchronize_rcu_expedited(void)
 {
 	synchronize_sched_expedited();
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

 /*
  * Because preemptable RCU does not exist, it never has any work to do.
  */
 static int rcu_preempt_pending(int cpu)
 {
 	return 0;
 }

 /*
  * Because preemptable RCU does not exist, it never needs any CPU.
  */
 static int rcu_preempt_needs_cpu(int cpu)
 {
 	return 0;
 }

 /*
  * Because preemptable RCU does not exist, rcu_barrier() is just
  * another name for rcu_barrier_sched().
  */
 void rcu_barrier(void)
 {
 	rcu_barrier_sched();
 }
 EXPORT_SYMBOL_GPL(rcu_barrier);

 /*
  * Because preemptable RCU does not exist, there is no per-CPU
  * data to initialize.
  */
 static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
 {
 }

 /*
  * Because there is no preemptable RCU, there are no callbacks to move.
  */
 static void rcu_preempt_send_cbs_to_orphanage(void)
 {
 }

 /*
  * Because preemptable RCU does not exist, it need not be initialized.
  */
 static void __init __rcu_init_preempt(void)
 {
 }

 #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
	/*
	* Read-Copy Update mechanism for mutual exclusion (tree-based version)
	* Internal non-public definitions that provide either classic
	* or preemptable semantics.
	*
	* 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 Red Hat, 2009
	* Copyright IBM Corporation, 2009
	*
	* Author: Ingo Molnar <mingo@elte.hu>
	* Paul E. McKenney <paulmck@linux.vnet.ibm.com>
	*/


	#ifdef CONFIG_TREE_PREEMPT_RCU

	struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt_state);
	DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);

	/*
	* Tell them what RCU they are running.
	*/
	static void rcu_bootup_announce(void)
	{
	printk(KERN_INFO
	"Experimental preemptable hierarchical RCU implementation.\n");
	}

	/*
	* Return the number of RCU-preempt batches processed thus far
	* for debug and statistics.
	*/
	long rcu_batches_completed_preempt(void)
	{
	return rcu_preempt_state.completed;
	}
	EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);

	/*
	* Return the number of RCU batches processed thus far for debug & stats.
	*/
	long rcu_batches_completed(void)
	{
	return rcu_batches_completed_preempt();
	}
	EXPORT_SYMBOL_GPL(rcu_batches_completed);

	/*
	* Record a preemptable-RCU quiescent state for the specified CPU. Note
	* that this just means that the task currently running on the CPU is
	* not in a quiescent state. There might be any number of tasks blocked
	* while in an RCU read-side critical section.
	*/
	static void rcu_preempt_qs(int cpu)
	{
	struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
	rdp->passed_quiesc_completed = rdp->completed;
	barrier();
	rdp->passed_quiesc = 1;
	}

	/*
	* We have entered the scheduler, and the current task might soon be
	* context-switched away from. If this task is in an RCU read-side
	* critical section, we will no longer be able to rely on the CPU to
	* record that fact, so we enqueue the task on the appropriate entry
	* of the blocked_tasks[] array. The task will dequeue itself when
	* it exits the outermost enclosing RCU read-side critical section.
	* Therefore, the current grace period cannot be permitted to complete
	* until the blocked_tasks[] entry indexed by the low-order bit of
	* rnp->gpnum empties.
	*
	* Caller must disable preemption.
	*/
	static void rcu_preempt_note_context_switch(int cpu)
	{
	struct task_struct *t = current;
	unsigned long flags;
	int phase;
	struct rcu_data *rdp;
	struct rcu_node *rnp;

	if (t->rcu_read_lock_nesting &&
	(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {

	/* Possibly blocking in an RCU read-side critical section. */
	rdp = rcu_preempt_state.rda[cpu];
	rnp = rdp->mynode;
	spin_lock_irqsave(&rnp->lock, flags);
	t->rcu_read_unlock_special \|= RCU_READ_UNLOCK_BLOCKED;
	t->rcu_blocked_node = rnp;

	/*
	* If this CPU has already checked in, then this task
	* will hold up the next grace period rather than the
	* current grace period. Queue the task accordingly.
	* If the task is queued for the current grace period
	* (i.e., this CPU has not yet passed through a quiescent
	* state for the current grace period), then as long
	* as that task remains queued, the current grace period
	* cannot end.
	*
	* But first, note that the current CPU must still be
	* on line!
	*/
	WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
	WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
	phase = (rnp->gpnum + !(rnp->qsmask & rdp->grpmask)) & 0x1;
	list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
	spin_unlock_irqrestore(&rnp->lock, flags);
	}

	/*
	* Either we were not in an RCU read-side critical section to
	* begin with, or we have now recorded that critical section
	* globally. Either way, we can now note a quiescent state
	* for this CPU. Again, if we were in an RCU read-side critical
	* section, and if that critical section was blocking the current
	* grace period, then the fact that the task has been enqueued
	* means that we continue to block the current grace period.
	*/
	rcu_preempt_qs(cpu);
	local_irq_save(flags);
	t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
	local_irq_restore(flags);
	}

	/*
	* Tree-preemptable RCU implementation for rcu_read_lock().
	* Just increment ->rcu_read_lock_nesting, shared state will be updated
	* if we block.
	*/
	void __rcu_read_lock(void)
	{
	ACCESS_ONCE(current->rcu_read_lock_nesting)++;
	barrier(); /* needed if we ever invoke rcu_read_lock in rcutree.c */
	}
	EXPORT_SYMBOL_GPL(__rcu_read_lock);

	/*
	* Check for preempted RCU readers blocking the current grace period
	* for the specified rcu_node structure. If the caller needs a reliable
	* answer, it must hold the rcu_node's ->lock.
	*/
	static int rcu_preempted_readers(struct rcu_node *rnp)
	{
	return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
	}

	static void rcu_read_unlock_special(struct task_struct *t)
	{
	int empty;
	unsigned long flags;
	unsigned long mask;
	struct rcu_node *rnp;
	int special;

	/* NMI handlers cannot block and cannot safely manipulate state. */
	if (in_nmi())
	return;

	local_irq_save(flags);

	/*
	* If RCU core is waiting for this CPU to exit critical section,
	* let it know that we have done so.
	*/
	special = t->rcu_read_unlock_special;
	if (special & RCU_READ_UNLOCK_NEED_QS) {
	t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
	rcu_preempt_qs(smp_processor_id());
	}

	/* Hardware IRQ handlers cannot block. */
	if (in_irq()) {
	local_irq_restore(flags);
	return;
	}

	/* Clean up if blocked during RCU read-side critical section. */
	if (special & RCU_READ_UNLOCK_BLOCKED) {
	t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;

	/*
	* Remove this task from the list it blocked on. The
	* task can migrate while we acquire the lock, but at
	* most one time. So at most two passes through loop.
	*/
	for (;;) {
	rnp = t->rcu_blocked_node;
	spin_lock(&rnp->lock); /* irqs already disabled. */
	if (rnp == t->rcu_blocked_node)
	break;
	spin_unlock(&rnp->lock); /* irqs remain disabled. */
	}
	empty = !rcu_preempted_readers(rnp);
	list_del_init(&t->rcu_node_entry);
	t->rcu_blocked_node = NULL;

	/*
	* If this was the last task on the current list, and if
	* we aren't waiting on any CPUs, report the quiescent state.
	* Note that both cpu_quiet_msk_finish() and cpu_quiet_msk()
	* drop rnp->lock and restore irq.
	*/
	if (!empty && rnp->qsmask == 0 &&
	!rcu_preempted_readers(rnp)) {
	struct rcu_node *rnp_p;

	if (rnp->parent == NULL) {
	/* Only one rcu_node in the tree. */
	cpu_quiet_msk_finish(&rcu_preempt_state, flags);
	return;
	}
	/* Report up the rest of the hierarchy. */
	mask = rnp->grpmask;
	spin_unlock_irqrestore(&rnp->lock, flags);
	rnp_p = rnp->parent;
	spin_lock_irqsave(&rnp_p->lock, flags);
	WARN_ON_ONCE(rnp->qsmask);
	cpu_quiet_msk(mask, &rcu_preempt_state, rnp_p, flags);
	return;
	}
	spin_unlock(&rnp->lock);
	}
	local_irq_restore(flags);
	}

	/*
	* Tree-preemptable RCU implementation for rcu_read_unlock().
	* Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
	* rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
	* invoke rcu_read_unlock_special() to clean up after a context switch
	* in an RCU read-side critical section and other special cases.
	*/
	void __rcu_read_unlock(void)
	{
	struct task_struct *t = current;

	barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */
	if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
	unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
	rcu_read_unlock_special(t);
	}
	EXPORT_SYMBOL_GPL(__rcu_read_unlock);

	#ifdef CONFIG_RCU_CPU_STALL_DETECTOR

	/*
	* Scan the current list of tasks blocked within RCU read-side critical
	* sections, printing out the tid of each.
	*/
	static void rcu_print_task_stall(struct rcu_node *rnp)
	{
	unsigned long flags;
	struct list_head *lp;
	int phase;
	struct task_struct *t;

	if (rcu_preempted_readers(rnp)) {
	spin_lock_irqsave(&rnp->lock, flags);
	phase = rnp->gpnum & 0x1;
	lp = &rnp->blocked_tasks[phase];
	list_for_each_entry(t, lp, rcu_node_entry)
	printk(" P%d", t->pid);
	spin_unlock_irqrestore(&rnp->lock, flags);
	}
	}

	#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

	/*
	* Check that the list of blocked tasks for the newly completed grace
	* period is in fact empty. It is a serious bug to complete a grace
	* period that still has RCU readers blocked! This function must be
	* invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
	* must be held by the caller.
	*/
	static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
	{
	WARN_ON_ONCE(rcu_preempted_readers(rnp));
	WARN_ON_ONCE(rnp->qsmask);
	}

	#ifdef CONFIG_HOTPLUG_CPU

	/*
	* Handle tasklist migration for case in which all CPUs covered by the
	* specified rcu_node have gone offline. Move them up to the root
	* rcu_node. The reason for not just moving them to the immediate
	* parent is to remove the need for rcu_read_unlock_special() to
	* make more than two attempts to acquire the target rcu_node's lock.
	*
	* Returns 1 if there was previously a task blocking the current grace
	* period on the specified rcu_node structure.
	*
	* The caller must hold rnp->lock with irqs disabled.
	*/
	static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
	struct rcu_node *rnp,
	struct rcu_data *rdp)
	{
	int i;
	struct list_head *lp;
	struct list_head *lp_root;
	int retval = rcu_preempted_readers(rnp);
	struct rcu_node *rnp_root = rcu_get_root(rsp);
	struct task_struct *tp;

	if (rnp == rnp_root) {
	WARN_ONCE(1, "Last CPU thought to be offlined?");
	return 0; /* Shouldn't happen: at least one CPU online. */
	}
	WARN_ON_ONCE(rnp != rdp->mynode &&
	(!list_empty(&rnp->blocked_tasks[0]) \|\|
	!list_empty(&rnp->blocked_tasks[1])));

	/*
	* Move tasks up to root rcu_node. Rely on the fact that the
	* root rcu_node can be at most one ahead of the rest of the
	* rcu_nodes in terms of gp_num value. This fact allows us to
	* move the blocked_tasks[] array directly, element by element.
	*/
	for (i = 0; i < 2; i++) {
	lp = &rnp->blocked_tasks[i];
	lp_root = &rnp_root->blocked_tasks[i];
	while (!list_empty(lp)) {
	tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
	spin_lock(&rnp_root->lock); /* irqs already disabled */
	list_del(&tp->rcu_node_entry);
	tp->rcu_blocked_node = rnp_root;
	list_add(&tp->rcu_node_entry, lp_root);
	spin_unlock(&rnp_root->lock); /* irqs remain disabled */
	}
	}

	return retval;
	}

	/*
	* Do CPU-offline processing for preemptable RCU.
	*/
	static void rcu_preempt_offline_cpu(int cpu)
	{
	__rcu_offline_cpu(cpu, &rcu_preempt_state);
	}

	#endif /* #ifdef CONFIG_HOTPLUG_CPU */

	/*
	* Check for a quiescent state from the current CPU. When a task blocks,
	* the task is recorded in the corresponding CPU's rcu_node structure,
	* which is checked elsewhere.
	*
	* Caller must disable hard irqs.
	*/
	static void rcu_preempt_check_callbacks(int cpu)
	{
	struct task_struct *t = current;

	if (t->rcu_read_lock_nesting == 0) {
	t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
	rcu_preempt_qs(cpu);
	return;
	}
	if (per_cpu(rcu_preempt_data, cpu).qs_pending)
	t->rcu_read_unlock_special \|= RCU_READ_UNLOCK_NEED_QS;
	}

	/*
	* Process callbacks for preemptable RCU.
	*/
	static void rcu_preempt_process_callbacks(void)
	{
	__rcu_process_callbacks(&rcu_preempt_state,
	&__get_cpu_var(rcu_preempt_data));
	}

	/*
	* Queue a preemptable-RCU callback for invocation after a grace period.
	*/
	void call_rcu(struct rcu_head head, void (func)(struct rcu_head *rcu))
	{
	__call_rcu(head, func, &rcu_preempt_state);
	}
	EXPORT_SYMBOL_GPL(call_rcu);

	/*
	* Wait for an rcu-preempt grace period. We are supposed to expedite the
	* grace period, but this is the crude slow compatability hack, so just
	* invoke synchronize_rcu().
	*/
	void synchronize_rcu_expedited(void)
	{
	synchronize_rcu();
	}
	EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

	/*
	* Check to see if there is any immediate preemptable-RCU-related work
	* to be done.
	*/
	static int rcu_preempt_pending(int cpu)
	{
	return __rcu_pending(&rcu_preempt_state,
	&per_cpu(rcu_preempt_data, cpu));
	}

	/*
	* Does preemptable RCU need the CPU to stay out of dynticks mode?
	*/
	static int rcu_preempt_needs_cpu(int cpu)
	{
	return !!per_cpu(rcu_preempt_data, cpu).nxtlist;
	}

	/**
	* rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
	*/
	void rcu_barrier(void)
	{
	_rcu_barrier(&rcu_preempt_state, call_rcu);
	}
	EXPORT_SYMBOL_GPL(rcu_barrier);

	/*
	* Initialize preemptable RCU's per-CPU data.
	*/
	static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
	{
	rcu_init_percpu_data(cpu, &rcu_preempt_state, 1);
	}

	/*
	* Move preemptable RCU's callbacks to ->orphan_cbs_list.
	*/
	static void rcu_preempt_send_cbs_to_orphanage(void)
	{
	rcu_send_cbs_to_orphanage(&rcu_preempt_state);
	}

	/*
	* Initialize preemptable RCU's state structures.
	*/
	static void __init __rcu_init_preempt(void)
	{
	RCU_INIT_FLAVOR(&rcu_preempt_state, rcu_preempt_data);
	}

	/*
	* Check for a task exiting while in a preemptable-RCU read-side
	* critical section, clean up if so. No need to issue warnings,
	* as debug_check_no_locks_held() already does this if lockdep
	* is enabled.
	*/
	void exit_rcu(void)
	{
	struct task_struct *t = current;

	if (t->rcu_read_lock_nesting == 0)
	return;
	t->rcu_read_lock_nesting = 1;
	rcu_read_unlock();
	}

	#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */

	/*
	* Tell them what RCU they are running.
	*/
	static void rcu_bootup_announce(void)
	{
	printk(KERN_INFO "Hierarchical RCU implementation.\n");
	}

	/*
	* Return the number of RCU batches processed thus far for debug & stats.
	*/
	long rcu_batches_completed(void)
	{
	return rcu_batches_completed_sched();
	}
	EXPORT_SYMBOL_GPL(rcu_batches_completed);

	/*
	* Because preemptable RCU does not exist, we never have to check for
	* CPUs being in quiescent states.
	*/
	static void rcu_preempt_note_context_switch(int cpu)
	{
	}

	/*
	* Because preemptable RCU does not exist, there are never any preempted
	* RCU readers.
	*/
	static int rcu_preempted_readers(struct rcu_node *rnp)
	{
	return 0;
	}

	#ifdef CONFIG_RCU_CPU_STALL_DETECTOR

	/*
	* Because preemptable RCU does not exist, we never have to check for
	* tasks blocked within RCU read-side critical sections.
	*/
	static void rcu_print_task_stall(struct rcu_node *rnp)
	{
	}

	#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

	/*
	* Because there is no preemptable RCU, there can be no readers blocked,
	* so there is no need to check for blocked tasks. So check only for
	* bogus qsmask values.
	*/
	static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
	{
	WARN_ON_ONCE(rnp->qsmask);
	}

	#ifdef CONFIG_HOTPLUG_CPU

	/*
	* Because preemptable RCU does not exist, it never needs to migrate
	* tasks that were blocked within RCU read-side critical sections, and
	* such non-existent tasks cannot possibly have been blocking the current
	* grace period.
	*/
	static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
	struct rcu_node *rnp,
	struct rcu_data *rdp)
	{
	return 0;
	}

	/*
	* Because preemptable RCU does not exist, it never needs CPU-offline
	* processing.
	*/
	static void rcu_preempt_offline_cpu(int cpu)
	{
	}

	#endif /* #ifdef CONFIG_HOTPLUG_CPU */

	/*
	* Because preemptable RCU does not exist, it never has any callbacks
	* to check.
	*/
	static void rcu_preempt_check_callbacks(int cpu)
	{
	}

	/*
	* Because preemptable RCU does not exist, it never has any callbacks
	* to process.
	*/
	static void rcu_preempt_process_callbacks(void)
	{
	}

	/*
	* In classic RCU, call_rcu() is just call_rcu_sched().
	*/
	void call_rcu(struct rcu_head head, void (func)(struct rcu_head *rcu))
	{
	call_rcu_sched(head, func);
	}
	EXPORT_SYMBOL_GPL(call_rcu);

	/*
	* Wait for an rcu-preempt grace period, but make it happen quickly.
	* But because preemptable RCU does not exist, map to rcu-sched.
	*/
	void synchronize_rcu_expedited(void)
	{
	synchronize_sched_expedited();
	}
	EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

	/*
	* Because preemptable RCU does not exist, it never has any work to do.
	*/
	static int rcu_preempt_pending(int cpu)
	{
	return 0;
	}

	/*
	* Because preemptable RCU does not exist, it never needs any CPU.
	*/
	static int rcu_preempt_needs_cpu(int cpu)
	{
	return 0;
	}

	/*
	* Because preemptable RCU does not exist, rcu_barrier() is just
	* another name for rcu_barrier_sched().
	*/
	void rcu_barrier(void)
	{
	rcu_barrier_sched();
	}
	EXPORT_SYMBOL_GPL(rcu_barrier);

	/*
	* Because preemptable RCU does not exist, there is no per-CPU
	* data to initialize.
	*/
	static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
	{
	}

	/*
	* Because there is no preemptable RCU, there are no callbacks to move.
	*/
	static void rcu_preempt_send_cbs_to_orphanage(void)
	{
	}

	/*
	* Because preemptable RCU does not exist, it need not be initialized.
	*/
	static void __init __rcu_init_preempt(void)
	{
	}

	#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */