kernel/rcu/srcutiny.c

   1 // SPDX-License-Identifier: GPL-2.0+
   2 /*
   3  * Sleepable Read-Copy Update mechanism for mutual exclusion,
   4  *      tiny version for non-preemptible single-CPU use.
   5  *
   6  * Copyright (C) IBM Corporation, 2017
   7  *
   8  * Author: Paul McKenney <paulmck@linux.ibm.com>
   9  */
  10
  11 #include <linux/export.h>
  12 #include <linux/mutex.h>
  13 #include <linux/preempt.h>
  14 #include <linux/rcupdate_wait.h>
  15 #include <linux/sched.h>
  16 #include <linux/delay.h>
  17 #include <linux/srcu.h>
  18
  19 #include <linux/rcu_node_tree.h>
  20 #include "rcu_segcblist.h"
  21 #include "rcu.h"
  22
  23 int rcu_scheduler_active __read_mostly;
  24 static LIST_HEAD(srcu_boot_list);
  25 static bool srcu_init_done;
  26
  27 static int init_srcu_struct_fields(struct srcu_struct *ssp)
  28 {
  29         ssp->srcu_lock_nesting[0] = 0;
  30         ssp->srcu_lock_nesting[1] = 0;
  31         init_swait_queue_head(&ssp->srcu_wq);
  32         ssp->srcu_cb_head = NULL;
  33         ssp->srcu_cb_tail = &ssp->srcu_cb_head;
  34         ssp->srcu_gp_running = false;
  35         ssp->srcu_gp_waiting = false;
  36         ssp->srcu_idx = 0;
  37         ssp->srcu_idx_max = 0;
  38         INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
  39         INIT_LIST_HEAD(&ssp->srcu_work.entry);
  40         return 0;
  41 }
  42
  43 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  44
  45 int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
  46                        struct lock_class_key *key)
  47 {
  48         /* Don't re-initialize a lock while it is held. */
  49         debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
  50         lockdep_init_map(&ssp->dep_map, name, key, 0);
  51         return init_srcu_struct_fields(ssp);
  52 }
  53 EXPORT_SYMBOL_GPL(__init_srcu_struct);
  54
  55 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  56
  57 /*
  58  * init_srcu_struct - initialize a sleep-RCU structure
  59  * @ssp: structure to initialize.
  60  *
  61  * Must invoke this on a given srcu_struct before passing that srcu_struct
  62  * to any other function.  Each srcu_struct represents a separate domain
  63  * of SRCU protection.
  64  */
  65 int init_srcu_struct(struct srcu_struct *ssp)
  66 {
  67         return init_srcu_struct_fields(ssp);
  68 }
  69 EXPORT_SYMBOL_GPL(init_srcu_struct);
  70
  71 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  72
  73 /*
  74  * cleanup_srcu_struct - deconstruct a sleep-RCU structure
  75  * @ssp: structure to clean up.
  76  *
  77  * Must invoke this after you are finished using a given srcu_struct that
  78  * was initialized via init_srcu_struct(), else you leak memory.
  79  */
  80 void cleanup_srcu_struct(struct srcu_struct *ssp)
  81 {
  82         WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
  83         flush_work(&ssp->srcu_work);
  84         WARN_ON(ssp->srcu_gp_running);
  85         WARN_ON(ssp->srcu_gp_waiting);
  86         WARN_ON(ssp->srcu_cb_head);
  87         WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
  88         WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
  89         WARN_ON(ssp->srcu_idx & 0x1);
  90 }
  91 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
  92
  93 /*
  94  * Removes the count for the old reader from the appropriate element of
  95  * the srcu_struct.
  96  */
  97 void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
  98 {
  99         int newval;
 100
 101         preempt_disable();  // Needed for PREEMPT_AUTO
 102         newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1;
 103         WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
 104         preempt_enable();
 105         if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task())
 106                 swake_up_one(&ssp->srcu_wq);
 107 }
 108 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 109
 110 /*
 111  * Workqueue handler to drive one grace period and invoke any callbacks
 112  * that become ready as a result.  Single-CPU and !PREEMPTION operation
 113  * means that we get away with murder on synchronization.  ;-)
 114  */
 115 void srcu_drive_gp(struct work_struct *wp)
 116 {
 117         int idx;
 118         struct rcu_head *lh;
 119         struct rcu_head *rhp;
 120         struct srcu_struct *ssp;
 121
 122         ssp = container_of(wp, struct srcu_struct, srcu_work);
 123         preempt_disable();  // Needed for PREEMPT_AUTO
 124         if (ssp->srcu_gp_running || ULONG_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) {
 125                 preempt_enable();
 126                 return; /* Already running or nothing to do. */
 127         }
 128
 129         /* Remove recently arrived callbacks and wait for readers. */
 130         WRITE_ONCE(ssp->srcu_gp_running, true);
 131         local_irq_disable();
 132         lh = ssp->srcu_cb_head;
 133         ssp->srcu_cb_head = NULL;
 134         ssp->srcu_cb_tail = &ssp->srcu_cb_head;
 135         local_irq_enable();
 136         idx = (ssp->srcu_idx & 0x2) / 2;
 137         WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 138         WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
 139         preempt_enable();
 140         swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
 141         preempt_disable();  // Needed for PREEMPT_AUTO
 142         WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
 143         WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 144         preempt_enable();
 145
 146         /* Invoke the callbacks we removed above. */
 147         while (lh) {
 148                 rhp = lh;
 149                 lh = lh->next;
 150                 debug_rcu_head_callback(rhp);
 151                 local_bh_disable();
 152                 rhp->func(rhp);
 153                 local_bh_enable();
 154         }
 155
 156         /*
 157          * Enable rescheduling, and if there are more callbacks,
 158          * reschedule ourselves.  This can race with a call_srcu()
 159          * at interrupt level, but the ->srcu_gp_running checks will
 160          * straighten that out.
 161          */
 162         preempt_disable();  // Needed for PREEMPT_AUTO
 163         WRITE_ONCE(ssp->srcu_gp_running, false);
 164         idx = ULONG_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max));
 165         preempt_enable();
 166         if (idx)
 167                 schedule_work(&ssp->srcu_work);
 168 }
 169 EXPORT_SYMBOL_GPL(srcu_drive_gp);
 170
 171 static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
 172 {
 173         unsigned long cookie;
 174
 175         preempt_disable();  // Needed for PREEMPT_AUTO
 176         cookie = get_state_synchronize_srcu(ssp);
 177         if (ULONG_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) {
 178                 preempt_enable();
 179                 return;
 180         }
 181         WRITE_ONCE(ssp->srcu_idx_max, cookie);
 182         if (!READ_ONCE(ssp->srcu_gp_running)) {
 183                 if (likely(srcu_init_done))
 184                         schedule_work(&ssp->srcu_work);
 185                 else if (list_empty(&ssp->srcu_work.entry))
 186                         list_add(&ssp->srcu_work.entry, &srcu_boot_list);
 187         }
 188         preempt_enable();
 189 }
 190
 191 /*
 192  * Enqueue an SRCU callback on the specified srcu_struct structure,
 193  * initiating grace-period processing if it is not already running.
 194  */
 195 void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 196                rcu_callback_t func)
 197 {
 198         unsigned long flags;
 199
 200         rhp->func = func;
 201         rhp->next = NULL;
 202         preempt_disable();  // Needed for PREEMPT_AUTO
 203         local_irq_save(flags);
 204         *ssp->srcu_cb_tail = rhp;
 205         ssp->srcu_cb_tail = &rhp->next;
 206         local_irq_restore(flags);
 207         srcu_gp_start_if_needed(ssp);
 208         preempt_enable();
 209 }
 210 EXPORT_SYMBOL_GPL(call_srcu);
 211
 212 /*
 213  * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 214  */
 215 void synchronize_srcu(struct srcu_struct *ssp)
 216 {
 217         struct rcu_synchronize rs;
 218
 219         srcu_lock_sync(&ssp->dep_map);
 220
 221         RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
 222                         lock_is_held(&rcu_bh_lock_map) ||
 223                         lock_is_held(&rcu_lock_map) ||
 224                         lock_is_held(&rcu_sched_lock_map),
 225                         "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
 226
 227         if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
 228                 return;
 229
 230         might_sleep();
 231         init_rcu_head_on_stack(&rs.head);
 232         init_completion(&rs.completion);
 233         call_srcu(ssp, &rs.head, wakeme_after_rcu);
 234         wait_for_completion(&rs.completion);
 235         destroy_rcu_head_on_stack(&rs.head);
 236 }
 237 EXPORT_SYMBOL_GPL(synchronize_srcu);
 238
 239 /*
 240  * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
 241  */
 242 unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
 243 {
 244         unsigned long ret;
 245
 246         barrier();
 247         ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
 248         barrier();
 249         return ret;
 250 }
 251 EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
 252
 253 /*
 254  * start_poll_synchronize_srcu - Provide cookie and start grace period
 255  *
 256  * The difference between this and get_state_synchronize_srcu() is that
 257  * this function ensures that the poll_state_synchronize_srcu() will
 258  * eventually return the value true.
 259  */
 260 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
 261 {
 262         unsigned long ret;
 263
 264         preempt_disable();  // Needed for PREEMPT_AUTO
 265         ret = get_state_synchronize_srcu(ssp);
 266         srcu_gp_start_if_needed(ssp);
 267         preempt_enable();
 268         return ret;
 269 }
 270 EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
 271
 272 /*
 273  * poll_state_synchronize_srcu - Has cookie's grace period ended?
 274  */
 275 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
 276 {
 277         unsigned long cur_s = READ_ONCE(ssp->srcu_idx);
 278
 279         barrier();
 280         return cookie == SRCU_GET_STATE_COMPLETED ||
 281                ULONG_CMP_GE(cur_s, cookie) || ULONG_CMP_LT(cur_s, cookie - 3);
 282 }
 283 EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
 284
 285 /* Lockdep diagnostics.  */
 286 void __init rcu_scheduler_starting(void)
 287 {
 288         rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
 289 }
 290
 291 /*
 292  * Queue work for srcu_struct structures with early boot callbacks.
 293  * The work won't actually execute until the workqueue initialization
 294  * phase that takes place after the scheduler starts.
 295  */
 296 void __init srcu_init(void)
 297 {
 298         struct srcu_struct *ssp;
 299
 300         srcu_init_done = true;
 301         while (!list_empty(&srcu_boot_list)) {
 302                 ssp = list_first_entry(&srcu_boot_list,
 303                                       struct srcu_struct, srcu_work.entry);
 304                 list_del_init(&ssp->srcu_work.entry);
 305                 schedule_work(&ssp->srcu_work);
 306         }
 307 }