text: // SPDX-License-Identifier: GPL-2.0+
  /*
   * Sleepable Read-Copy Update mechanism for mutual exclusion,
   *	tiny version for non-preemptible single-CPU use.
   *
   * Copyright (C) IBM Corporation, 2017
   *
   * Author: Paul McKenney <paulmck@linux.ibm.com>
   */
  
  #include <linux/export.h>
  #include <linux/mutex.h>
  #include <linux/preempt.h>
  #include <linux/rcupdate_wait.h>
  #include <linux/sched.h>
  #include <linux/delay.h>
  #include <linux/srcu.h>
  
  #include <linux/rcu_node_tree.h>
  #include "rcu_segcblist.h"
  #include "rcu.h"
  
  int rcu_scheduler_active __read_mostly;
  static LIST_HEAD(srcu_boot_list);
  static bool srcu_init_done;
  
  static int init_srcu_struct_fields(struct srcu_struct *ssp)
  {
  	ssp->srcu_lock_nesting[0] = 0;
  	ssp->srcu_lock_nesting[1] = 0;
  	init_swait_queue_head(&ssp->srcu_wq);
  	ssp->srcu_cb_head = NULL;
  	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
  	ssp->srcu_gp_running = false;
  	ssp->srcu_gp_waiting = false;
  	ssp->srcu_idx = 0;
  	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
  	INIT_LIST_HEAD(&ssp->srcu_work.entry);
  	return 0;
  }
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  
  int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
  		       struct lock_class_key *key)
  {
  	/* Don't re-initialize a lock while it is held. */
  	debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
  	lockdep_init_map(&ssp->dep_map, name, key, 0);
  	return init_srcu_struct_fields(ssp);
  }
  EXPORT_SYMBOL_GPL(__init_srcu_struct);
  
  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  
  /*
   * init_srcu_struct - initialize a sleep-RCU structure
   * @ssp: structure to initialize.
   *
   * Must invoke this on a given srcu_struct before passing that srcu_struct
   * to any other function.  Each srcu_struct represents a separate domain
   * of SRCU protection.
   */
  int init_srcu_struct(struct srcu_struct *ssp)
  {
  	return init_srcu_struct_fields(ssp);
  }
  EXPORT_SYMBOL_GPL(init_srcu_struct);
  
  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  
  /*
   * cleanup_srcu_struct - deconstruct a sleep-RCU structure
   * @ssp: structure to clean up.
   *
   * Must invoke this after you are finished using a given srcu_struct that
   * was initialized via init_srcu_struct(), else you leak memory.
   */
  void cleanup_srcu_struct(struct srcu_struct *ssp)
  {
  	WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
  	flush_work(&ssp->srcu_work);
  	WARN_ON(ssp->srcu_gp_running);
  	WARN_ON(ssp->srcu_gp_waiting);
  	WARN_ON(ssp->srcu_cb_head);
  	WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
  }
  EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
  
  /*
   * Removes the count for the old reader from the appropriate element of
   * the srcu_struct.
   */
  void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
  {
  	int newval = ssp->srcu_lock_nesting[idx] - 1;
  
  	WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
  	if (!newval && READ_ONCE(ssp->srcu_gp_waiting))
  		swake_up_one(&ssp->srcu_wq);
  }
  EXPORT_SYMBOL_GPL(__srcu_read_unlock);
  
  /*
   * Workqueue handler to drive one grace period and invoke any callbacks
   * that become ready as a result.  Single-CPU and !PREEMPT operation
   * means that we get away with murder on synchronization.  ;-)
   */
  void srcu_drive_gp(struct work_struct *wp)
  {
  	int idx;
  	struct rcu_head *lh;
  	struct rcu_head *rhp;
  	struct srcu_struct *ssp;
  
  	ssp = container_of(wp, struct srcu_struct, srcu_work);
  	if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
  		return; /* Already running or nothing to do. */
  
  	/* Remove recently arrived callbacks and wait for readers. */
  	WRITE_ONCE(ssp->srcu_gp_running, true);
  	local_irq_disable();
  	lh = ssp->srcu_cb_head;
  	ssp->srcu_cb_head = NULL;
  	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
  	local_irq_enable();
  	idx = ssp->srcu_idx;
  	WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx);
  	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
  	swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
  	WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
  
  	/* Invoke the callbacks we removed above. */
  	while (lh) {
  		rhp = lh;
  		lh = lh->next;
  		local_bh_disable();
  		rhp->func(rhp);
  		local_bh_enable();
  	}
  
  	/*
  	 * Enable rescheduling, and if there are more callbacks,
  	 * reschedule ourselves.  This can race with a call_srcu()
  	 * at interrupt level, but the ->srcu_gp_running checks will
  	 * straighten that out.
  	 */
  	WRITE_ONCE(ssp->srcu_gp_running, false);
  	if (READ_ONCE(ssp->srcu_cb_head))
  		schedule_work(&ssp->srcu_work);
  }
  EXPORT_SYMBOL_GPL(srcu_drive_gp);
  
  /*
   * Enqueue an SRCU callback on the specified srcu_struct structure,
   * initiating grace-period processing if it is not already running.
   */
  void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
  	       rcu_callback_t func)
  {
  	unsigned long flags;
  
  	rhp->func = func;
  	rhp->next = NULL;
  	local_irq_save(flags);
  	*ssp->srcu_cb_tail = rhp;
  	ssp->srcu_cb_tail = &rhp->next;
  	local_irq_restore(flags);
  	if (!READ_ONCE(ssp->srcu_gp_running)) {
  		if (likely(srcu_init_done))
  			schedule_work(&ssp->srcu_work);
  		else if (list_empty(&ssp->srcu_work.entry))
  			list_add(&ssp->srcu_work.entry, &srcu_boot_list);
  	}
  }
  EXPORT_SYMBOL_GPL(call_srcu);
  
  /*
   * synchronize_srcu - wait for prior SRCU read-side critical-section completion
   */
  void synchronize_srcu(struct srcu_struct *ssp)
  {
  	struct rcu_synchronize rs;
  
  	init_rcu_head_on_stack(&rs.head);
  	init_completion(&rs.completion);
  	call_srcu(ssp, &rs.head, wakeme_after_rcu);
  	wait_for_completion(&rs.completion);
  	destroy_rcu_head_on_stack(&rs.head);
  }
  EXPORT_SYMBOL_GPL(synchronize_srcu);
  
  /* Lockdep diagnostics.  */
  void __init rcu_scheduler_starting(void)
  {
  	rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
  }
  
  /*
   * Queue work for srcu_struct structures with early boot callbacks.
   * The work won't actually execute until the workqueue initialization
   * phase that takes place after the scheduler starts.
   */
  void __init srcu_init(void)
  {
  	struct srcu_struct *ssp;
  
  	srcu_init_done = true;
  	while (!list_empty(&srcu_boot_list)) {
  		ssp = list_first_entry(&srcu_boot_list,
  				      struct srcu_struct, srcu_work.entry);
  		list_del_init(&ssp->srcu_work.entry);
  		schedule_work(&ssp->srcu_work);
  	}
  }
  