FreeBSD/Linux Kernel Cross Reference
sys/kernel/rcutiny_plugin.h

     /*
      * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition
      * Internal non-public definitions that provide either classic
      * or preemptible 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 (c) 2010 Linaro
     *
     * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
     */
    
    #include <linux/kthread.h>
    #include <linux/module.h>
    #include <linux/debugfs.h>
    #include <linux/seq_file.h>
    
    /* Global control variables for rcupdate callback mechanism. */
    struct rcu_ctrlblk {
            struct rcu_head *rcucblist;     /* List of pending callbacks (CBs). */
            struct rcu_head **donetail;     /* ->next pointer of last "done" CB. */
            struct rcu_head **curtail;      /* ->next pointer of last CB. */
            RCU_TRACE(long qlen);           /* Number of pending CBs. */
            RCU_TRACE(char *name);          /* Name of RCU type. */
    };
    
    /* Definition for rcupdate control block. */
    static struct rcu_ctrlblk rcu_sched_ctrlblk = {
            .donetail       = &rcu_sched_ctrlblk.rcucblist,
            .curtail        = &rcu_sched_ctrlblk.rcucblist,
            RCU_TRACE(.name = "rcu_sched")
    };
    
    static struct rcu_ctrlblk rcu_bh_ctrlblk = {
            .donetail       = &rcu_bh_ctrlblk.rcucblist,
            .curtail        = &rcu_bh_ctrlblk.rcucblist,
            RCU_TRACE(.name = "rcu_bh")
    };
    
    #ifdef CONFIG_DEBUG_LOCK_ALLOC
    int rcu_scheduler_active __read_mostly;
    EXPORT_SYMBOL_GPL(rcu_scheduler_active);
    #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
    
    #ifdef CONFIG_TINY_PREEMPT_RCU
    
    #include <linux/delay.h>
    
    /* Global control variables for preemptible RCU. */
    struct rcu_preempt_ctrlblk {
            struct rcu_ctrlblk rcb; /* curtail: ->next ptr of last CB for GP. */
            struct rcu_head **nexttail;
                                    /* Tasks blocked in a preemptible RCU */
                                    /*  read-side critical section while an */
                                    /*  preemptible-RCU grace period is in */
                                    /*  progress must wait for a later grace */
                                    /*  period.  This pointer points to the */
                                    /*  ->next pointer of the last task that */
                                    /*  must wait for a later grace period, or */
                                    /*  to &->rcb.rcucblist if there is no */
                                    /*  such task. */
            struct list_head blkd_tasks;
                                    /* Tasks blocked in RCU read-side critical */
                                    /*  section.  Tasks are placed at the head */
                                    /*  of this list and age towards the tail. */
            struct list_head *gp_tasks;
                                    /* Pointer to the first task blocking the */
                                    /*  current grace period, or NULL if there */
                                    /*  is no such task. */
            struct list_head *exp_tasks;
                                    /* Pointer to first task blocking the */
                                    /*  current expedited grace period, or NULL */
                                    /*  if there is no such task.  If there */
                                    /*  is no current expedited grace period, */
                                    /*  then there cannot be any such task. */
    #ifdef CONFIG_RCU_BOOST
            struct list_head *boost_tasks;
                                    /* Pointer to first task that needs to be */
                                    /*  priority-boosted, or NULL if no priority */
                                    /*  boosting is needed.  If there is no */
                                    /*  current or expedited grace period, there */
                                    /*  can be no such task. */
    #endif /* #ifdef CONFIG_RCU_BOOST */
            u8 gpnum;               /* Current grace period. */
            u8 gpcpu;               /* Last grace period blocked by the CPU. */
            u8 completed;           /* Last grace period completed. */
                                    /*  If all three are equal, RCU is idle. */
   #ifdef CONFIG_RCU_BOOST
           unsigned long boost_time; /* When to start boosting (jiffies) */
   #endif /* #ifdef CONFIG_RCU_BOOST */
   #ifdef CONFIG_RCU_TRACE
           unsigned long n_grace_periods;
   #ifdef CONFIG_RCU_BOOST
           unsigned long n_tasks_boosted;
                                   /* Total number of tasks boosted. */
           unsigned long n_exp_boosts;
                                   /* Number of tasks boosted for expedited GP. */
           unsigned long n_normal_boosts;
                                   /* Number of tasks boosted for normal GP. */
           unsigned long n_balk_blkd_tasks;
                                   /* Refused to boost: no blocked tasks. */
           unsigned long n_balk_exp_gp_tasks;
                                   /* Refused to boost: nothing blocking GP. */
           unsigned long n_balk_boost_tasks;
                                   /* Refused to boost: already boosting. */
           unsigned long n_balk_notyet;
                                   /* Refused to boost: not yet time. */
           unsigned long n_balk_nos;
                                   /* Refused to boost: not sure why, though. */
                                   /*  This can happen due to race conditions. */
   #endif /* #ifdef CONFIG_RCU_BOOST */
   #endif /* #ifdef CONFIG_RCU_TRACE */
   };
   
   static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
           .rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
           .rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
           .nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
           .blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),
           RCU_TRACE(.rcb.name = "rcu_preempt")
   };
   
   static int rcu_preempted_readers_exp(void);
   static void rcu_report_exp_done(void);
   
   /*
    * Return true if the CPU has not yet responded to the current grace period.
    */
   static int rcu_cpu_blocking_cur_gp(void)
   {
           return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
   }
   
   /*
    * Check for a running RCU reader.  Because there is only one CPU,
    * there can be but one running RCU reader at a time.  ;-)
    *
    * Returns zero if there are no running readers.  Returns a positive
    * number if there is at least one reader within its RCU read-side
    * critical section.  Returns a negative number if an outermost reader
    * is in the midst of exiting from its RCU read-side critical section
    *
    * Returns zero if there are no running readers.  Returns a positive
    * number if there is at least one reader within its RCU read-side
    * critical section.  Returns a negative number if an outermost reader
    * is in the midst of exiting from its RCU read-side critical section.
    */
   static int rcu_preempt_running_reader(void)
   {
           return current->rcu_read_lock_nesting;
   }
   
   /*
    * Check for preempted RCU readers blocking any grace period.
    * If the caller needs a reliable answer, it must disable hard irqs.
    */
   static int rcu_preempt_blocked_readers_any(void)
   {
           return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
   }
   
   /*
    * Check for preempted RCU readers blocking the current grace period.
    * If the caller needs a reliable answer, it must disable hard irqs.
    */
   static int rcu_preempt_blocked_readers_cgp(void)
   {
           return rcu_preempt_ctrlblk.gp_tasks != NULL;
   }
   
   /*
    * Return true if another preemptible-RCU grace period is needed.
    */
   static int rcu_preempt_needs_another_gp(void)
   {
           return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
   }
   
   /*
    * Return true if a preemptible-RCU grace period is in progress.
    * The caller must disable hardirqs.
    */
   static int rcu_preempt_gp_in_progress(void)
   {
           return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
   }
   
   /*
    * Advance a ->blkd_tasks-list pointer to the next entry, instead
    * returning NULL if at the end of the list.
    */
   static struct list_head *rcu_next_node_entry(struct task_struct *t)
   {
           struct list_head *np;
   
           np = t->rcu_node_entry.next;
           if (np == &rcu_preempt_ctrlblk.blkd_tasks)
                   np = NULL;
           return np;
   }
   
   #ifdef CONFIG_RCU_TRACE
   
   #ifdef CONFIG_RCU_BOOST
   static void rcu_initiate_boost_trace(void);
   #endif /* #ifdef CONFIG_RCU_BOOST */
   
   /*
    * Dump additional statistice for TINY_PREEMPT_RCU.
    */
   static void show_tiny_preempt_stats(struct seq_file *m)
   {
           seq_printf(m, "rcu_preempt: qlen=%ld gp=%lu g%u/p%u/c%u tasks=%c%c%c\n",
                      rcu_preempt_ctrlblk.rcb.qlen,
                      rcu_preempt_ctrlblk.n_grace_periods,
                      rcu_preempt_ctrlblk.gpnum,
                      rcu_preempt_ctrlblk.gpcpu,
                      rcu_preempt_ctrlblk.completed,
                      "T."[list_empty(&rcu_preempt_ctrlblk.blkd_tasks)],
                      "N."[!rcu_preempt_ctrlblk.gp_tasks],
                      "E."[!rcu_preempt_ctrlblk.exp_tasks]);
   #ifdef CONFIG_RCU_BOOST
           seq_printf(m, "%sttb=%c ntb=%lu neb=%lu nnb=%lu j=%04x bt=%04x\n",
                      "             ",
                      "B."[!rcu_preempt_ctrlblk.boost_tasks],
                      rcu_preempt_ctrlblk.n_tasks_boosted,
                      rcu_preempt_ctrlblk.n_exp_boosts,
                      rcu_preempt_ctrlblk.n_normal_boosts,
                      (int)(jiffies & 0xffff),
                      (int)(rcu_preempt_ctrlblk.boost_time & 0xffff));
           seq_printf(m, "%s: nt=%lu egt=%lu bt=%lu ny=%lu nos=%lu\n",
                      "             balk",
                      rcu_preempt_ctrlblk.n_balk_blkd_tasks,
                      rcu_preempt_ctrlblk.n_balk_exp_gp_tasks,
                      rcu_preempt_ctrlblk.n_balk_boost_tasks,
                      rcu_preempt_ctrlblk.n_balk_notyet,
                      rcu_preempt_ctrlblk.n_balk_nos);
   #endif /* #ifdef CONFIG_RCU_BOOST */
   }
   
   #endif /* #ifdef CONFIG_RCU_TRACE */
   
   #ifdef CONFIG_RCU_BOOST
   
   #include "rtmutex_common.h"
   
   #define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
   
   /* Controls for rcu_kthread() kthread. */
   static struct task_struct *rcu_kthread_task;
   static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
   static unsigned long have_rcu_kthread_work;
   
   /*
    * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
    * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
    */
   static int rcu_boost(void)
   {
           unsigned long flags;
           struct rt_mutex mtx;
           struct task_struct *t;
           struct list_head *tb;
   
           if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
               rcu_preempt_ctrlblk.exp_tasks == NULL)
                   return 0;  /* Nothing to boost. */
   
           local_irq_save(flags);
   
           /*
            * Recheck with irqs disabled: all tasks in need of boosting
            * might exit their RCU read-side critical sections on their own
            * if we are preempted just before disabling irqs.
            */
           if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
               rcu_preempt_ctrlblk.exp_tasks == NULL) {
                   local_irq_restore(flags);
                   return 0;
           }
   
           /*
            * Preferentially boost tasks blocking expedited grace periods.
            * This cannot starve the normal grace periods because a second
            * expedited grace period must boost all blocked tasks, including
            * those blocking the pre-existing normal grace period.
            */
           if (rcu_preempt_ctrlblk.exp_tasks != NULL) {
                   tb = rcu_preempt_ctrlblk.exp_tasks;
                   RCU_TRACE(rcu_preempt_ctrlblk.n_exp_boosts++);
           } else {
                   tb = rcu_preempt_ctrlblk.boost_tasks;
                   RCU_TRACE(rcu_preempt_ctrlblk.n_normal_boosts++);
           }
           RCU_TRACE(rcu_preempt_ctrlblk.n_tasks_boosted++);
   
           /*
            * We boost task t by manufacturing an rt_mutex that appears to
            * be held by task t.  We leave a pointer to that rt_mutex where
            * task t can find it, and task t will release the mutex when it
            * exits its outermost RCU read-side critical section.  Then
            * simply acquiring this artificial rt_mutex will boost task
            * t's priority.  (Thanks to tglx for suggesting this approach!)
            */
           t = container_of(tb, struct task_struct, rcu_node_entry);
           rt_mutex_init_proxy_locked(&mtx, t);
           t->rcu_boost_mutex = &mtx;
           local_irq_restore(flags);
           rt_mutex_lock(&mtx);
           rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
   
           return ACCESS_ONCE(rcu_preempt_ctrlblk.boost_tasks) != NULL ||
                  ACCESS_ONCE(rcu_preempt_ctrlblk.exp_tasks) != NULL;
   }
   
   /*
    * Check to see if it is now time to start boosting RCU readers blocking
    * the current grace period, and, if so, tell the rcu_kthread_task to
    * start boosting them.  If there is an expedited boost in progress,
    * we wait for it to complete.
    *
    * If there are no blocked readers blocking the current grace period,
    * return 0 to let the caller know, otherwise return 1.  Note that this
    * return value is independent of whether or not boosting was done.
    */
   static int rcu_initiate_boost(void)
   {
           if (!rcu_preempt_blocked_readers_cgp() &&
               rcu_preempt_ctrlblk.exp_tasks == NULL) {
                   RCU_TRACE(rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++);
                   return 0;
           }
           if (rcu_preempt_ctrlblk.exp_tasks != NULL ||
               (rcu_preempt_ctrlblk.gp_tasks != NULL &&
                rcu_preempt_ctrlblk.boost_tasks == NULL &&
                ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))) {
                   if (rcu_preempt_ctrlblk.exp_tasks == NULL)
                           rcu_preempt_ctrlblk.boost_tasks =
                                   rcu_preempt_ctrlblk.gp_tasks;
                   invoke_rcu_callbacks();
           } else {
                   RCU_TRACE(rcu_initiate_boost_trace());
           }
           return 1;
   }
   
   #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
   
   /*
    * Do priority-boost accounting for the start of a new grace period.
    */
   static void rcu_preempt_boost_start_gp(void)
   {
           rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
   }
   
   #else /* #ifdef CONFIG_RCU_BOOST */
   
   /*
    * If there is no RCU priority boosting, we don't initiate boosting,
    * but we do indicate whether there are blocked readers blocking the
    * current grace period.
    */
   static int rcu_initiate_boost(void)
   {
           return rcu_preempt_blocked_readers_cgp();
   }
   
   /*
    * If there is no RCU priority boosting, nothing to do at grace-period start.
    */
   static void rcu_preempt_boost_start_gp(void)
   {
   }
   
   #endif /* else #ifdef CONFIG_RCU_BOOST */
   
   /*
    * Record a preemptible-RCU quiescent state for the specified CPU.  Note
    * that this just means that the task currently running on the CPU is
    * in a quiescent state.  There might be any number of tasks blocked
    * while in an RCU read-side critical section.
    *
    * Unlike the other rcu_*_qs() functions, callers to this function
    * must disable irqs in order to protect the assignment to
    * ->rcu_read_unlock_special.
    *
    * Because this is a single-CPU implementation, the only way a grace
    * period can end is if the CPU is in a quiescent state.  The reason is
    * that a blocked preemptible-RCU reader can exit its critical section
    * only if the CPU is running it at the time.  Therefore, when the
    * last task blocking the current grace period exits its RCU read-side
    * critical section, neither the CPU nor blocked tasks will be stopping
    * the current grace period.  (In contrast, SMP implementations
    * might have CPUs running in RCU read-side critical sections that
    * block later grace periods -- but this is not possible given only
    * one CPU.)
    */
   static void rcu_preempt_cpu_qs(void)
   {
           /* Record both CPU and task as having responded to current GP. */
           rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
           current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
   
           /* If there is no GP then there is nothing more to do.  */
           if (!rcu_preempt_gp_in_progress())
                   return;
           /*
            * Check up on boosting.  If there are readers blocking the
            * current grace period, leave.
            */
           if (rcu_initiate_boost())
                   return;
   
           /* Advance callbacks. */
           rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
           rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.rcb.curtail;
           rcu_preempt_ctrlblk.rcb.curtail = rcu_preempt_ctrlblk.nexttail;
   
           /* If there are no blocked readers, next GP is done instantly. */
           if (!rcu_preempt_blocked_readers_any())
                   rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.nexttail;
   
           /* If there are done callbacks, cause them to be invoked. */
           if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
                   invoke_rcu_callbacks();
   }
   
   /*
    * Start a new RCU grace period if warranted.  Hard irqs must be disabled.
    */
   static void rcu_preempt_start_gp(void)
   {
           if (!rcu_preempt_gp_in_progress() && rcu_preempt_needs_another_gp()) {
   
                   /* Official start of GP. */
                   rcu_preempt_ctrlblk.gpnum++;
                   RCU_TRACE(rcu_preempt_ctrlblk.n_grace_periods++);
   
                   /* Any blocked RCU readers block new GP. */
                   if (rcu_preempt_blocked_readers_any())
                           rcu_preempt_ctrlblk.gp_tasks =
                                   rcu_preempt_ctrlblk.blkd_tasks.next;
   
                   /* Set up for RCU priority boosting. */
                   rcu_preempt_boost_start_gp();
   
                   /* If there is no running reader, CPU is done with GP. */
                   if (!rcu_preempt_running_reader())
                           rcu_preempt_cpu_qs();
           }
   }
   
   /*
    * 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 blkd_tasks list.
    * If the task started after the current grace period began, as recorded
    * by ->gpcpu, we enqueue at the beginning of the list.  Otherwise
    * before the element referenced by ->gp_tasks (or at the tail if
    * ->gp_tasks is NULL) and point ->gp_tasks at the newly added element.
    * 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 ->gp_tasks pointer becomes
    * NULL.
    *
    * Caller must disable preemption.
    */
   void rcu_preempt_note_context_switch(void)
   {
           struct task_struct *t = current;
           unsigned long flags;
   
           local_irq_save(flags); /* must exclude scheduler_tick(). */
           if (rcu_preempt_running_reader() > 0 &&
               (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
   
                   /* Possibly blocking in an RCU read-side critical section. */
                   t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
   
                   /*
                    * 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.
                    */
                   list_add(&t->rcu_node_entry, &rcu_preempt_ctrlblk.blkd_tasks);
                   if (rcu_cpu_blocking_cur_gp())
                           rcu_preempt_ctrlblk.gp_tasks = &t->rcu_node_entry;
           } else if (rcu_preempt_running_reader() < 0 &&
                      t->rcu_read_unlock_special) {
                   /*
                    * Complete exit from RCU read-side critical section on
                    * behalf of preempted instance of __rcu_read_unlock().
                    */
                   rcu_read_unlock_special(t);
           }
   
           /*
            * 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 current grace period continues to be blocked.
            */
           rcu_preempt_cpu_qs();
           local_irq_restore(flags);
   }
   
   /*
    * Handle special cases during rcu_read_unlock(), such as needing to
    * notify RCU core processing or task having blocked during the RCU
    * read-side critical section.
    */
   void rcu_read_unlock_special(struct task_struct *t)
   {
           int empty;
           int empty_exp;
           unsigned long flags;
           struct list_head *np;
   #ifdef CONFIG_RCU_BOOST
           struct rt_mutex *rbmp = NULL;
   #endif /* #ifdef CONFIG_RCU_BOOST */
           int special;
   
           /*
            * NMI handlers cannot block and cannot safely manipulate state.
            * They therefore cannot possibly be special, so just leave.
            */
           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)
                   rcu_preempt_cpu_qs();
   
           /* Hardware IRQ handlers cannot block. */
           if (in_irq() || in_serving_softirq()) {
                   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 ->blkd_tasks list and adjust
                    * any pointers that might have been referencing it.
                    */
                   empty = !rcu_preempt_blocked_readers_cgp();
                   empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
                   np = rcu_next_node_entry(t);
                   list_del_init(&t->rcu_node_entry);
                   if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
                           rcu_preempt_ctrlblk.gp_tasks = np;
                   if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
                           rcu_preempt_ctrlblk.exp_tasks = np;
   #ifdef CONFIG_RCU_BOOST
                   if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
                           rcu_preempt_ctrlblk.boost_tasks = np;
   #endif /* #ifdef CONFIG_RCU_BOOST */
   
                   /*
                    * If this was the last task on the current list, and if
                    * we aren't waiting on the CPU, report the quiescent state
                    * and start a new grace period if needed.
                    */
                   if (!empty && !rcu_preempt_blocked_readers_cgp()) {
                           rcu_preempt_cpu_qs();
                           rcu_preempt_start_gp();
                   }
   
                   /*
                    * If this was the last task on the expedited lists,
                    * then we need wake up the waiting task.
                    */
                   if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
                           rcu_report_exp_done();
           }
   #ifdef CONFIG_RCU_BOOST
           /* Unboost self if was boosted. */
           if (t->rcu_boost_mutex != NULL) {
                   rbmp = t->rcu_boost_mutex;
                   t->rcu_boost_mutex = NULL;
                   rt_mutex_unlock(rbmp);
           }
   #endif /* #ifdef CONFIG_RCU_BOOST */
           local_irq_restore(flags);
   }
   
   /*
    * Check for a quiescent state from the current CPU.  When a task blocks,
    * the task is recorded in the rcu_preempt_ctrlblk structure, which is
    * checked elsewhere.  This is called from the scheduling-clock interrupt.
    *
    * Caller must disable hard irqs.
    */
   static void rcu_preempt_check_callbacks(void)
   {
           struct task_struct *t = current;
   
           if (rcu_preempt_gp_in_progress() &&
               (!rcu_preempt_running_reader() ||
                !rcu_cpu_blocking_cur_gp()))
                   rcu_preempt_cpu_qs();
           if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
               rcu_preempt_ctrlblk.rcb.donetail)
                   invoke_rcu_callbacks();
           if (rcu_preempt_gp_in_progress() &&
               rcu_cpu_blocking_cur_gp() &&
               rcu_preempt_running_reader() > 0)
                   t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
   }
   
   /*
    * TINY_PREEMPT_RCU has an extra callback-list tail pointer to
    * update, so this is invoked from rcu_process_callbacks() to
    * handle that case.  Of course, it is invoked for all flavors of
    * RCU, but RCU callbacks can appear only on one of the lists, and
    * neither ->nexttail nor ->donetail can possibly be NULL, so there
    * is no need for an explicit check.
    */
   static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
   {
           if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
                   rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
   }
   
   /*
    * Process callbacks for preemptible RCU.
    */
   static void rcu_preempt_process_callbacks(void)
   {
           __rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
   }
   
   /*
    * Queue a preemptible -RCU callback for invocation after a grace period.
    */
   void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
   {
           unsigned long flags;
   
           debug_rcu_head_queue(head);
           head->func = func;
           head->next = NULL;
   
           local_irq_save(flags);
           *rcu_preempt_ctrlblk.nexttail = head;
           rcu_preempt_ctrlblk.nexttail = &head->next;
           RCU_TRACE(rcu_preempt_ctrlblk.rcb.qlen++);
           rcu_preempt_start_gp();  /* checks to see if GP needed. */
           local_irq_restore(flags);
   }
   EXPORT_SYMBOL_GPL(call_rcu);
   
   /*
    * synchronize_rcu - wait until a grace period has elapsed.
    *
    * Control will return to the caller some time after a full grace
    * period has elapsed, 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 synchronize_rcu(void)
   {
           rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
                              !lock_is_held(&rcu_lock_map) &&
                              !lock_is_held(&rcu_sched_lock_map),
                              "Illegal synchronize_rcu() in RCU read-side critical section");
   
   #ifdef CONFIG_DEBUG_LOCK_ALLOC
           if (!rcu_scheduler_active)
                   return;
   #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
   
           WARN_ON_ONCE(rcu_preempt_running_reader());
           if (!rcu_preempt_blocked_readers_any())
                   return;
   
           /* Once we get past the fastpath checks, same code as rcu_barrier(). */
           if (rcu_expedited)
                   synchronize_rcu_expedited();
           else
                   rcu_barrier();
   }
   EXPORT_SYMBOL_GPL(synchronize_rcu);
   
   static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
   static unsigned long sync_rcu_preempt_exp_count;
   static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
   
   /*
    * Return non-zero if there are any tasks in RCU read-side critical
    * sections blocking the current preemptible-RCU expedited grace period.
    * If there is no preemptible-RCU expedited grace period currently in
    * progress, returns zero unconditionally.
    */
   static int rcu_preempted_readers_exp(void)
   {
           return rcu_preempt_ctrlblk.exp_tasks != NULL;
   }
   
   /*
    * Report the exit from RCU read-side critical section for the last task
    * that queued itself during or before the current expedited preemptible-RCU
    * grace period.
    */
   static void rcu_report_exp_done(void)
   {
           wake_up(&sync_rcu_preempt_exp_wq);
   }
   
   /*
    * Wait for an rcu-preempt grace period, but expedite it.  The basic idea
    * is to rely in the fact that there is but one CPU, and that it is
    * illegal for a task to invoke synchronize_rcu_expedited() while in a
    * preemptible-RCU read-side critical section.  Therefore, any such
    * critical sections must correspond to blocked tasks, which must therefore
    * be on the ->blkd_tasks list.  So just record the current head of the
    * list in the ->exp_tasks pointer, and wait for all tasks including and
    * after the task pointed to by ->exp_tasks to drain.
    */
   void synchronize_rcu_expedited(void)
   {
           unsigned long flags;
           struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
           unsigned long snap;
   
           barrier(); /* ensure prior action seen before grace period. */
   
           WARN_ON_ONCE(rcu_preempt_running_reader());
   
           /*
            * Acquire lock so that there is only one preemptible RCU grace
            * period in flight.  Of course, if someone does the expedited
            * grace period for us while we are acquiring the lock, just leave.
            */
           snap = sync_rcu_preempt_exp_count + 1;
           mutex_lock(&sync_rcu_preempt_exp_mutex);
           if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
                   goto unlock_mb_ret; /* Others did our work for us. */
   
           local_irq_save(flags);
   
           /*
            * All RCU readers have to already be on blkd_tasks because
            * we cannot legally be executing in an RCU read-side critical
            * section.
            */
   
           /* Snapshot current head of ->blkd_tasks list. */
           rpcp->exp_tasks = rpcp->blkd_tasks.next;
           if (rpcp->exp_tasks == &rpcp->blkd_tasks)
                   rpcp->exp_tasks = NULL;
   
           /* Wait for tail of ->blkd_tasks list to drain. */
           if (!rcu_preempted_readers_exp()) {
                   local_irq_restore(flags);
           } else {
                   rcu_initiate_boost();
                   local_irq_restore(flags);
                   wait_event(sync_rcu_preempt_exp_wq,
                              !rcu_preempted_readers_exp());
           }
   
           /* Clean up and exit. */
           barrier(); /* ensure expedited GP seen before counter increment. */
           sync_rcu_preempt_exp_count++;
   unlock_mb_ret:
           mutex_unlock(&sync_rcu_preempt_exp_mutex);
           barrier(); /* ensure subsequent action seen after grace period. */
   }
   EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
   
   /*
    * Does preemptible RCU need the CPU to stay out of dynticks mode?
    */
   int rcu_preempt_needs_cpu(void)
   {
           return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
   }
   
   #else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
   
   #ifdef CONFIG_RCU_TRACE
   
   /*
    * Because preemptible RCU does not exist, it is not necessary to
    * dump out its statistics.
    */
   static void show_tiny_preempt_stats(struct seq_file *m)
   {
   }
   
   #endif /* #ifdef CONFIG_RCU_TRACE */
   
   /*
    * Because preemptible RCU does not exist, it never has any callbacks
    * to check.
    */
   static void rcu_preempt_check_callbacks(void)
   {
   }
   
   /*
    * Because preemptible RCU does not exist, it never has any callbacks
    * to remove.
    */
   static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
   {
   }
   
   /*
    * Because preemptible RCU does not exist, it never has any callbacks
    * to process.
    */
   static void rcu_preempt_process_callbacks(void)
   {
   }
   
   #endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */
   
   #ifdef CONFIG_RCU_BOOST
   
   /*
    * Wake up rcu_kthread() to process callbacks now eligible for invocation
    * or to boost readers.
    */
   static void invoke_rcu_callbacks(void)
   {
           have_rcu_kthread_work = 1;
           if (rcu_kthread_task != NULL)
                   wake_up(&rcu_kthread_wq);
   }
   
   #ifdef CONFIG_RCU_TRACE
   
   /*
    * Is the current CPU running the RCU-callbacks kthread?
    * Caller must have preemption disabled.
    */
   static bool rcu_is_callbacks_kthread(void)
   {
           return rcu_kthread_task == current;
   }
   
   #endif /* #ifdef CONFIG_RCU_TRACE */
   
   /*
    * This kthread invokes RCU callbacks whose grace periods have
    * elapsed.  It is awakened as needed, and takes the place of the
    * RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
    * This is a kthread, but it is never stopped, at least not until
    * the system goes down.
    */
   static int rcu_kthread(void *arg)
   {
           unsigned long work;
           unsigned long morework;
           unsigned long flags;
   
           for (;;) {
                   wait_event_interruptible(rcu_kthread_wq,
                                            have_rcu_kthread_work != 0);
                   morework = rcu_boost();
                   local_irq_save(flags);
                   work = have_rcu_kthread_work;
                   have_rcu_kthread_work = morework;
                   local_irq_restore(flags);
                   if (work)
                           rcu_process_callbacks(NULL);
                   schedule_timeout_interruptible(1); /* Leave CPU for others. */
           }
   
           return 0;  /* Not reached, but needed to shut gcc up. */
   }
   
   /*
    * Spawn the kthread that invokes RCU callbacks.
    */
   static int __init rcu_spawn_kthreads(void)
   {
           struct sched_param sp;
   
           rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
           sp.sched_priority = RCU_BOOST_PRIO;
           sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
           return 0;
   }
   early_initcall(rcu_spawn_kthreads);
   
   #else /* #ifdef CONFIG_RCU_BOOST */
   
   /* Hold off callback invocation until early_initcall() time. */
   static int rcu_scheduler_fully_active __read_mostly;
   
   /*
    * Start up softirq processing of callbacks.
    */
   void invoke_rcu_callbacks(void)
   {
           if (rcu_scheduler_fully_active)
                   raise_softirq(RCU_SOFTIRQ);
   }
   
   #ifdef CONFIG_RCU_TRACE
   
   /*
    * There is no callback kthread, so this thread is never it.
    */
   static bool rcu_is_callbacks_kthread(void)
   {
           return false;
   }
   
   #endif /* #ifdef CONFIG_RCU_TRACE */
   
   static int __init rcu_scheduler_really_started(void)
   {
           rcu_scheduler_fully_active = 1;
           open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
           raise_softirq(RCU_SOFTIRQ);  /* Invoke any callbacks from early boot. */
           return 0;
   }
   early_initcall(rcu_scheduler_really_started);
   
   #endif /* #else #ifdef CONFIG_RCU_BOOST */
   
   #ifdef CONFIG_DEBUG_LOCK_ALLOC
   #include <linux/kernel_stat.h>
   
   /*
    * During boot, we forgive RCU lockdep issues.  After this function is
    * invoked, we start taking RCU lockdep issues seriously.
    */
   void __init rcu_scheduler_starting(void)
   {
           WARN_ON(nr_context_switches() > 0);
           rcu_scheduler_active = 1;
   }
   
   #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
   
   #ifdef CONFIG_RCU_TRACE
   
   #ifdef CONFIG_RCU_BOOST
   
   static void rcu_initiate_boost_trace(void)
   {
           if (list_empty(&rcu_preempt_ctrlblk.blkd_tasks))
                   rcu_preempt_ctrlblk.n_balk_blkd_tasks++;
           else if (rcu_preempt_ctrlblk.gp_tasks == NULL &&
                    rcu_preempt_ctrlblk.exp_tasks == NULL)
                   rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++;
           else if (rcu_preempt_ctrlblk.boost_tasks != NULL)
                   rcu_preempt_ctrlblk.n_balk_boost_tasks++;
           else if (!ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))
                   rcu_preempt_ctrlblk.n_balk_notyet++;
           else
                   rcu_preempt_ctrlblk.n_balk_nos++;
   }
   
   #endif /* #ifdef CONFIG_RCU_BOOST */
   
   static void rcu_trace_sub_qlen(struct rcu_ctrlblk *rcp, int n)
   {
           unsigned long flags;
   
           local_irq_save(flags);
           rcp->qlen -= n;
           local_irq_restore(flags);
  }
  
  /*
   * Dump statistics for TINY_RCU, such as they are.
   */
  static int show_tiny_stats(struct seq_file *m, void *unused)
  {
          show_tiny_preempt_stats(m);
          seq_printf(m, "rcu_sched: qlen: %ld\n", rcu_sched_ctrlblk.qlen);
          seq_printf(m, "rcu_bh: qlen: %ld\n", rcu_bh_ctrlblk.qlen);
          return 0;
  }
  
  static int show_tiny_stats_open(struct inode *inode, struct file *file)
  {
          return single_open(file, show_tiny_stats, NULL);
  }
  
  static const struct file_operations show_tiny_stats_fops = {
          .owner = THIS_MODULE,
          .open = show_tiny_stats_open,
          .read = seq_read,
          .llseek = seq_lseek,
          .release = single_release,
  };
  
  static struct dentry *rcudir;
  
  static int __init rcutiny_trace_init(void)
  {
          struct dentry *retval;
  
          rcudir = debugfs_create_dir("rcu", NULL);
          if (!rcudir)
                  goto free_out;
          retval = debugfs_create_file("rcudata", 0444, rcudir,
                                       NULL, &show_tiny_stats_fops);
          if (!retval)
                  goto free_out;
          return 0;
  free_out:
          debugfs_remove_recursive(rcudir);
          return 1;
  }
  
  static void __exit rcutiny_trace_cleanup(void)
  {
          debugfs_remove_recursive(rcudir);
  }
  
  module_init(rcutiny_trace_init);
  module_exit(rcutiny_trace_cleanup);
  
  MODULE_AUTHOR("Paul E. McKenney");
  MODULE_DESCRIPTION("Read-Copy Update tracing for tiny implementation");
  MODULE_LICENSE("GPL");
  
  #endif /* #ifdef CONFIG_RCU_TRACE */

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