FreeBSD/Linux Kernel Cross Reference
sys/kernel/rcuclassic.c

     /*
      * 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
     *
     */
    #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>
    
    #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_ctrlblk rcu_ctrlblk = {
            .cur = -300,
            .completed = -300,
            .pending = -300,
            .lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
            .cpumask = CPU_MASK_NONE,
    };
    static struct rcu_ctrlblk rcu_bh_ctrlblk = {
            .cur = -300,
            .completed = -300,
            .pending = -300,
            .lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
            .cpumask = CPU_MASK_NONE,
    };
    
    DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
    DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
    
    static int blimit = 10;
    static int qhimark = 10000;
    static int qlowmark = 100;
    
    #ifdef CONFIG_SMP
    static void force_quiescent_state(struct rcu_data *rdp,
                            struct rcu_ctrlblk *rcp)
    {
            int cpu;
            cpumask_t cpumask;
            unsigned long flags;
    
            set_need_resched();
            spin_lock_irqsave(&rcp->lock, flags);
            if (unlikely(!rcp->signaled)) {
                    rcp->signaled = 1;
                    /*
                     * Don't send IPI to itself. With irqs disabled,
                     * rdp->cpu is the current cpu.
                     *
                     * cpu_online_map is updated by the _cpu_down()
                    * using __stop_machine(). Since we're in irqs disabled
                    * section, __stop_machine() is not exectuting, hence
                    * the cpu_online_map is stable.
                    *
                    * However,  a cpu might have been offlined _just_ before
                    * we disabled irqs while entering here.
                    * And rcu subsystem might not yet have handled the CPU_DEAD
                    * notification, leading to the offlined cpu's bit
                    * being set in the rcp->cpumask.
                    *
                    * Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent
                    * sending smp_reschedule() to an offlined CPU.
                    */
                   cpus_and(cpumask, rcp->cpumask, cpu_online_map);
                   cpu_clear(rdp->cpu, cpumask);
                   for_each_cpu_mask_nr(cpu, cpumask)
                           smp_send_reschedule(cpu);
           }
           spin_unlock_irqrestore(&rcp->lock, flags);
   }
   #else
   static inline void force_quiescent_state(struct rcu_data *rdp,
                           struct rcu_ctrlblk *rcp)
   {
           set_need_resched();
   }
   #endif
   
   static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp,
                   struct rcu_data *rdp)
   {
           long batch;
   
           head->next = NULL;
           smp_mb(); /* Read of rcu->cur must happen after any change by caller. */
   
           /*
            * Determine the batch number of this callback.
            *
            * Using ACCESS_ONCE to avoid the following error when gcc eliminates
            * local variable "batch" and emits codes like this:
            *      1) rdp->batch = rcp->cur + 1 # gets old value
            *      ......
            *      2)rcu_batch_after(rcp->cur + 1, rdp->batch) # gets new value
            * then [*nxttail[0], *nxttail[1]) may contain callbacks
            * that batch# = rdp->batch, see the comment of struct rcu_data.
            */
           batch = ACCESS_ONCE(rcp->cur) + 1;
   
           if (rdp->nxtlist && rcu_batch_after(batch, rdp->batch)) {
                   /* process callbacks */
                   rdp->nxttail[0] = rdp->nxttail[1];
                   rdp->nxttail[1] = rdp->nxttail[2];
                   if (rcu_batch_after(batch - 1, rdp->batch))
                           rdp->nxttail[0] = rdp->nxttail[2];
           }
   
           rdp->batch = batch;
           *rdp->nxttail[2] = head;
           rdp->nxttail[2] = &head->next;
   
           if (unlikely(++rdp->qlen > qhimark)) {
                   rdp->blimit = INT_MAX;
                   force_quiescent_state(rdp, &rcu_ctrlblk);
           }
   }
   
   #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
   
   static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
   {
           rcp->gp_start = jiffies;
           rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
   }
   
   static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
   {
           int cpu;
           long delta;
           unsigned long flags;
   
           /* Only let one CPU complain about others per time interval. */
   
           spin_lock_irqsave(&rcp->lock, flags);
           delta = jiffies - rcp->jiffies_stall;
           if (delta < 2 || rcp->cur != rcp->completed) {
                   spin_unlock_irqrestore(&rcp->lock, flags);
                   return;
           }
           rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
           spin_unlock_irqrestore(&rcp->lock, flags);
   
           /* OK, time to rat on our buddy... */
   
           printk(KERN_ERR "RCU detected CPU stalls:");
           for_each_possible_cpu(cpu) {
                   if (cpu_isset(cpu, rcp->cpumask))
                           printk(" %d", cpu);
           }
           printk(" (detected by %d, t=%ld jiffies)\n",
                  smp_processor_id(), (long)(jiffies - rcp->gp_start));
   }
   
   static void print_cpu_stall(struct rcu_ctrlblk *rcp)
   {
           unsigned long flags;
   
           printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu jiffies)\n",
                           smp_processor_id(), jiffies,
                           jiffies - rcp->gp_start);
           dump_stack();
           spin_lock_irqsave(&rcp->lock, flags);
           if ((long)(jiffies - rcp->jiffies_stall) >= 0)
                   rcp->jiffies_stall =
                           jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
           spin_unlock_irqrestore(&rcp->lock, flags);
           set_need_resched();  /* kick ourselves to get things going. */
   }
   
   static void check_cpu_stall(struct rcu_ctrlblk *rcp)
   {
           long delta;
   
           delta = jiffies - rcp->jiffies_stall;
           if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0) {
   
                   /* We haven't checked in, so go dump stack. */
                   print_cpu_stall(rcp);
   
           } else if (rcp->cur != rcp->completed && delta >= 2) {
   
                   /* They had two seconds to dump stack, so complain. */
                   print_other_cpu_stall(rcp);
           }
   }
   
   #else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
   
   static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
   {
   }
   
   static inline void check_cpu_stall(struct rcu_ctrlblk *rcp)
   {
   }
   
   #endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
   
   /**
    * 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_ctrlblk, &__get_cpu_var(rcu_data));
           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_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
           local_irq_restore(flags);
   }
   EXPORT_SYMBOL_GPL(call_rcu_bh);
   
   /*
    * Return the number of RCU batches processed thus far.  Useful
    * for debug and statistics.
    */
   long rcu_batches_completed(void)
   {
           return rcu_ctrlblk.completed;
   }
   EXPORT_SYMBOL_GPL(rcu_batches_completed);
   
   /*
    * Return the number of RCU batches processed thus far.  Useful
    * for debug and statistics.
    */
   long rcu_batches_completed_bh(void)
   {
           return rcu_bh_ctrlblk.completed;
   }
   EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
   
   /* Raises the softirq for processing rcu_callbacks. */
   static inline void raise_rcu_softirq(void)
   {
           raise_softirq(RCU_SOFTIRQ);
   }
   
   /*
    * Invoke the completed RCU callbacks. They are expected to be in
    * a per-cpu list.
    */
   static void rcu_do_batch(struct rcu_data *rdp)
   {
           unsigned long flags;
           struct rcu_head *next, *list;
           int count = 0;
   
           list = rdp->donelist;
           while (list) {
                   next = list->next;
                   prefetch(next);
                   list->func(list);
                   list = next;
                   if (++count >= rdp->blimit)
                           break;
           }
           rdp->donelist = list;
   
           local_irq_save(flags);
           rdp->qlen -= count;
           local_irq_restore(flags);
           if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
                   rdp->blimit = blimit;
   
           if (!rdp->donelist)
                   rdp->donetail = &rdp->donelist;
           else
                   raise_rcu_softirq();
   }
   
   /*
    * Grace period handling:
    * The grace period handling consists out of two steps:
    * - A new grace period is started.
    *   This is done by rcu_start_batch. The start is not broadcasted to
    *   all cpus, they must pick this up by comparing rcp->cur with
    *   rdp->quiescbatch. All cpus are recorded  in the
    *   rcu_ctrlblk.cpumask bitmap.
    * - All cpus must go through a quiescent state.
    *   Since the start of the grace period is not broadcasted, at least two
    *   calls to rcu_check_quiescent_state are required:
    *   The first call just notices that a new grace period is running. The
    *   following calls check if there was a quiescent state since the beginning
    *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
    *   the bitmap is empty, then the grace period is completed.
    *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
    *   period (if necessary).
    */
   
   /*
    * Register a new batch of callbacks, and start it up if there is currently no
    * active batch and the batch to be registered has not already occurred.
    * Caller must hold rcu_ctrlblk.lock.
    */
   static void rcu_start_batch(struct rcu_ctrlblk *rcp)
   {
           if (rcp->cur != rcp->pending &&
                           rcp->completed == rcp->cur) {
                   rcp->cur++;
                   record_gp_stall_check_time(rcp);
   
                   /*
                    * 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(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
   
                   rcp->signaled = 0;
           }
   }
   
   /*
    * cpu went through a quiescent state since the beginning of the grace period.
    * Clear it from the cpu mask and complete the grace period if it was the last
    * cpu. Start another grace period if someone has further entries pending
    */
   static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
   {
           cpu_clear(cpu, rcp->cpumask);
           if (cpus_empty(rcp->cpumask)) {
                   /* batch completed ! */
                   rcp->completed = rcp->cur;
                   rcu_start_batch(rcp);
           }
   }
   
   /*
    * Check if the cpu has gone through a quiescent state (say context
    * switch). If so and if it already hasn't done so in this RCU
    * quiescent cycle, then indicate that it has done so.
    */
   static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
                                           struct rcu_data *rdp)
   {
           unsigned long flags;
   
           if (rdp->quiescbatch != rcp->cur) {
                   /* start new grace period: */
                   rdp->qs_pending = 1;
                   rdp->passed_quiesc = 0;
                   rdp->quiescbatch = rcp->cur;
                   return;
           }
   
           /* Grace period already completed for this cpu?
            * qs_pending is checked instead of the actual bitmap to avoid
            * cacheline trashing.
            */
           if (!rdp->qs_pending)
                   return;
   
           /*
            * Was there a quiescent state since the beginning of the grace
            * period? If no, then exit and wait for the next call.
            */
           if (!rdp->passed_quiesc)
                   return;
           rdp->qs_pending = 0;
   
           spin_lock_irqsave(&rcp->lock, flags);
           /*
            * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
            * during cpu startup. Ignore the quiescent state.
            */
           if (likely(rdp->quiescbatch == rcp->cur))
                   cpu_quiet(rdp->cpu, rcp);
   
           spin_unlock_irqrestore(&rcp->lock, flags);
   }
   
   
   #ifdef CONFIG_HOTPLUG_CPU
   
   /* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
    * locking requirements, the list it's pulling from has to belong to a cpu
    * which is dead and hence not processing interrupts.
    */
   static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
                                   struct rcu_head **tail, long batch)
   {
           unsigned long flags;
   
           if (list) {
                   local_irq_save(flags);
                   this_rdp->batch = batch;
                   *this_rdp->nxttail[2] = list;
                   this_rdp->nxttail[2] = tail;
                   local_irq_restore(flags);
           }
   }
   
   static void __rcu_offline_cpu(struct rcu_data *this_rdp,
                                   struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
   {
           unsigned long flags;
   
           /*
            * if the cpu going offline owns the grace period
            * we can block indefinitely waiting for it, so flush
            * it here
            */
           spin_lock_irqsave(&rcp->lock, flags);
           if (rcp->cur != rcp->completed)
                   cpu_quiet(rdp->cpu, rcp);
           rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail, rcp->cur + 1);
           rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail[2], rcp->cur + 1);
           spin_unlock(&rcp->lock);
   
           this_rdp->qlen += rdp->qlen;
           local_irq_restore(flags);
   }
   
   static void rcu_offline_cpu(int cpu)
   {
           struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
           struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);
   
           __rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
                                           &per_cpu(rcu_data, cpu));
           __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
                                           &per_cpu(rcu_bh_data, cpu));
           put_cpu_var(rcu_data);
           put_cpu_var(rcu_bh_data);
   }
   
   #else
   
   static void rcu_offline_cpu(int cpu)
   {
   }
   
   #endif
   
   /*
    * This does the RCU processing work from softirq context.
    */
   static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
                                           struct rcu_data *rdp)
   {
           unsigned long flags;
           long completed_snap;
   
           if (rdp->nxtlist) {
                   local_irq_save(flags);
                   completed_snap = ACCESS_ONCE(rcp->completed);
   
                   /*
                    * move the other grace-period-completed entries to
                    * [rdp->nxtlist, *rdp->nxttail[0]) temporarily
                    */
                   if (!rcu_batch_before(completed_snap, rdp->batch))
                           rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2];
                   else if (!rcu_batch_before(completed_snap, rdp->batch - 1))
                           rdp->nxttail[0] = rdp->nxttail[1];
   
                   /*
                    * the grace period for entries in
                    * [rdp->nxtlist, *rdp->nxttail[0]) has completed and
                    * move these entries to donelist
                    */
                   if (rdp->nxttail[0] != &rdp->nxtlist) {
                           *rdp->donetail = rdp->nxtlist;
                           rdp->donetail = rdp->nxttail[0];
                           rdp->nxtlist = *rdp->nxttail[0];
                           *rdp->donetail = NULL;
   
                           if (rdp->nxttail[1] == rdp->nxttail[0])
                                   rdp->nxttail[1] = &rdp->nxtlist;
                           if (rdp->nxttail[2] == rdp->nxttail[0])
                                   rdp->nxttail[2] = &rdp->nxtlist;
                           rdp->nxttail[0] = &rdp->nxtlist;
                   }
   
                   local_irq_restore(flags);
   
                   if (rcu_batch_after(rdp->batch, rcp->pending)) {
                           unsigned long flags2;
   
                           /* and start it/schedule start if it's a new batch */
                           spin_lock_irqsave(&rcp->lock, flags2);
                           if (rcu_batch_after(rdp->batch, rcp->pending)) {
                                   rcp->pending = rdp->batch;
                                   rcu_start_batch(rcp);
                           }
                           spin_unlock_irqrestore(&rcp->lock, flags2);
                   }
           }
   
           rcu_check_quiescent_state(rcp, rdp);
           if (rdp->donelist)
                   rcu_do_batch(rdp);
   }
   
   static void rcu_process_callbacks(struct softirq_action *unused)
   {
           /*
            * Memory references from any prior RCU read-side critical sections
            * executed by the interrupted code must be see before any RCU
            * grace-period manupulations below.
            */
   
           smp_mb(); /* See above block comment. */
   
           __rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
           __rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
   
           /*
            * Memory references from any later RCU read-side critical sections
            * executed by the interrupted code must be see after any RCU
            * grace-period manupulations above.
            */
   
           smp_mb(); /* See above block comment. */
   }
   
   static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
   {
           /* Check for CPU stalls, if enabled. */
           check_cpu_stall(rcp);
   
           if (rdp->nxtlist) {
                   long completed_snap = ACCESS_ONCE(rcp->completed);
   
                   /*
                    * This cpu has pending rcu entries and the grace period
                    * for them has completed.
                    */
                   if (!rcu_batch_before(completed_snap, rdp->batch))
                           return 1;
                   if (!rcu_batch_before(completed_snap, rdp->batch - 1) &&
                                   rdp->nxttail[0] != rdp->nxttail[1])
                           return 1;
                   if (rdp->nxttail[0] != &rdp->nxtlist)
                           return 1;
   
                   /*
                    * This cpu has pending rcu entries and the new batch
                    * for then hasn't been started nor scheduled start
                    */
                   if (rcu_batch_after(rdp->batch, rcp->pending))
                           return 1;
           }
   
           /* This cpu has finished callbacks to invoke */
           if (rdp->donelist)
                   return 1;
   
           /* The rcu core waits for a quiescent state from the cpu */
           if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
                   return 1;
   
           /* nothing to do */
           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_ctrlblk, &per_cpu(rcu_data, cpu)) ||
                   __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, 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_data *rdp = &per_cpu(rcu_data, cpu);
           struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
   
           return !!rdp->nxtlist || !!rdp_bh->nxtlist || rcu_pending(cpu);
   }
   
   /*
    * 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.
    */
   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.
                    *
                    * Also do a memory barrier.  This is needed to handle
                    * the case where writes from a preempt-disable section
                    * of code get reordered into schedule() by this CPU's
                    * write buffer.  The memory barrier makes sure that
                    * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see
                    * by other CPUs to happen after any such write.
                    */
   
                   smp_mb();  /* See above block comment. */
                   rcu_qsctr_inc(cpu);
                   rcu_bh_qsctr_inc(cpu);
   
           } 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.  The memory barrier
                    * is needed for the same reason as is the above one.
                    */
   
                   smp_mb();  /* See above block comment. */
                   rcu_bh_qsctr_inc(cpu);
           }
           raise_rcu_softirq();
   }
   
   static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
                                                   struct rcu_data *rdp)
   {
           unsigned long flags;
   
           spin_lock_irqsave(&rcp->lock, flags);
           memset(rdp, 0, sizeof(*rdp));
           rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2] = &rdp->nxtlist;
           rdp->donetail = &rdp->donelist;
           rdp->quiescbatch = rcp->completed;
           rdp->qs_pending = 0;
           rdp->cpu = cpu;
           rdp->blimit = blimit;
           spin_unlock_irqrestore(&rcp->lock, flags);
   }
   
   static void __cpuinit rcu_online_cpu(int cpu)
   {
           struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
           struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);
   
           rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
           rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
           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)
   {
   #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
           printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
   #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
           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(blimit, int, 0);
   module_param(qhimark, int, 0);
   module_param(qlowmark, int, 0);

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