rcu: Simplify rcu_read_unlock_special() quiescent-state accounting

[safe/jmp/linux-2.6] / kernel / rcutree_plugin.h

/*
 * 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 inline 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);
                phase = !(rnp->qsmask & rdp->grpmask) ^ (rnp->gpnum & 0x1);
                list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
                smp_mb();  /* Ensure later ctxt swtch seen after above. */
                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);
        t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
}

/*
 * 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);

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);
                        if (rnp == t->rcu_blocked_node)
                                break;
                        spin_unlock(&rnp->lock);
                }
                empty = list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
                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 &&
                    list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1])) {
                        t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
                        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 = rnp->parent;
                        spin_lock_irqsave(&rnp->lock, flags);
                        cpu_quiet_msk(mask, &rcu_preempt_state, rnp, 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 = rnp->gpnum & 0x1;
        struct task_struct *t;

        if (!list_empty(&rnp->blocked_tasks[phase])) {
                spin_lock_irqsave(&rnp->lock, flags);
                phase = rnp->gpnum & 0x1; /* re-read under lock. */
                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(!list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]));
}

/*
 * Check for preempted RCU readers 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]);
}

#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.
 *
 * The caller must hold rnp->lock with irqs disabled.
 */
static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
                                      struct rcu_node *rnp)
{
        int i;
        struct list_head *lp;
        struct list_head *lp_root;
        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;  /* Shouldn't happen: at least one CPU online. */
        }

        /*
         * 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 */
                }
        }
}

/*
 * 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);

/*
 * 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;
}

/*
 * 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);
}

/*
 * 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 inline 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)
{
}

#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.
 */
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
}

/*
 * 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_HOTPLUG_CPU

/*
 * Because preemptable RCU does not exist, it never needs to migrate
 * tasks that were blocked within RCU read-side critical sections.
 */
static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
                                      struct rcu_node *rnp)
{
}

/*
 * 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.
 */
void rcu_preempt_check_callbacks(int cpu)
{
}

/*
 * Because preemptable RCU does not exist, it never has any callbacks
 * to process.
 */
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);

/*
 * 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, there is no per-CPU
 * data to initialize.
 */
static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
{
}

#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */