kernel/rcu/sync.c

   1 // SPDX-License-Identifier: GPL-2.0+
   2 /*
   3  * RCU-based infrastructure for lightweight reader-writer locking
   4  *
   5  * Copyright (c) 2015, Red Hat, Inc.
   6  *
   7  * Author: Oleg Nesterov <oleg@redhat.com>
   8  */
   9
  10 #include <linux/rcu_sync.h>
  11 #include <linux/sched.h>
  12
  13 enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY };
  14
  15 #define rss_lock        gp_wait.lock
  16
  17 /**
  18  * rcu_sync_init() - Initialize an rcu_sync structure
  19  * @rsp: Pointer to rcu_sync structure to be initialized
  20  */
  21 void rcu_sync_init(struct rcu_sync *rsp)
  22 {
  23         memset(rsp, 0, sizeof(*rsp));
  24         init_waitqueue_head(&rsp->gp_wait);
  25 }
  26
  27 /**
  28  * rcu_sync_enter_start - Force readers onto slow path for multiple updates
  29  * @rsp: Pointer to rcu_sync structure to use for synchronization
  30  *
  31  * Must be called after rcu_sync_init() and before first use.
  32  *
  33  * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
  34  * pairs turn into NO-OPs.
  35  */
  36 void rcu_sync_enter_start(struct rcu_sync *rsp)
  37 {
  38         rsp->gp_count++;
  39         rsp->gp_state = GP_PASSED;
  40 }
  41
  42
  43 static void rcu_sync_func(struct rcu_head *rhp);
  44
  45 static void rcu_sync_call(struct rcu_sync *rsp)
  46 {
  47         call_rcu(&rsp->cb_head, rcu_sync_func);
  48 }
  49
  50 /**
  51  * rcu_sync_func() - Callback function managing reader access to fastpath
  52  * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
  53  *
  54  * This function is passed to call_rcu() function by rcu_sync_enter() and
  55  * rcu_sync_exit(), so that it is invoked after a grace period following the
  56  * that invocation of enter/exit.
  57  *
  58  * If it is called by rcu_sync_enter() it signals that all the readers were
  59  * switched onto slow path.
  60  *
  61  * If it is called by rcu_sync_exit() it takes action based on events that
  62  * have taken place in the meantime, so that closely spaced rcu_sync_enter()
  63  * and rcu_sync_exit() pairs need not wait for a grace period.
  64  *
  65  * If another rcu_sync_enter() is invoked before the grace period
  66  * ended, reset state to allow the next rcu_sync_exit() to let the
  67  * readers back onto their fastpaths (after a grace period).  If both
  68  * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
  69  * before the grace period ended, re-invoke call_rcu() on behalf of that
  70  * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
  71  * can again use their fastpaths.
  72  */
  73 static void rcu_sync_func(struct rcu_head *rhp)
  74 {
  75         struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
  76         unsigned long flags;
  77
  78         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
  79         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
  80
  81         spin_lock_irqsave(&rsp->rss_lock, flags);
  82         if (rsp->gp_count) {
  83                 /*
  84                  * We're at least a GP after the GP_IDLE->GP_ENTER transition.
  85                  */
  86                 WRITE_ONCE(rsp->gp_state, GP_PASSED);
  87                 wake_up_locked(&rsp->gp_wait);
  88         } else if (rsp->gp_state == GP_REPLAY) {
  89                 /*
  90                  * A new rcu_sync_exit() has happened; requeue the callback to
  91                  * catch a later GP.
  92                  */
  93                 WRITE_ONCE(rsp->gp_state, GP_EXIT);
  94                 rcu_sync_call(rsp);
  95         } else {
  96                 /*
  97                  * We're at least a GP after the last rcu_sync_exit(); eveybody
  98                  * will now have observed the write side critical section.
  99                  * Let 'em rip!.
 100                  */
 101                 WRITE_ONCE(rsp->gp_state, GP_IDLE);
 102         }
 103         spin_unlock_irqrestore(&rsp->rss_lock, flags);
 104 }
 105
 106 /**
 107  * rcu_sync_enter() - Force readers onto slowpath
 108  * @rsp: Pointer to rcu_sync structure to use for synchronization
 109  *
 110  * This function is used by updaters who need readers to make use of
 111  * a slowpath during the update.  After this function returns, all
 112  * subsequent calls to rcu_sync_is_idle() will return false, which
 113  * tells readers to stay off their fastpaths.  A later call to
 114  * rcu_sync_exit() re-enables reader slowpaths.
 115  *
 116  * When called in isolation, rcu_sync_enter() must wait for a grace
 117  * period, however, closely spaced calls to rcu_sync_enter() can
 118  * optimize away the grace-period wait via a state machine implemented
 119  * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
 120  */
 121 void rcu_sync_enter(struct rcu_sync *rsp)
 122 {
 123         int gp_state;
 124
 125         spin_lock_irq(&rsp->rss_lock);
 126         gp_state = rsp->gp_state;
 127         if (gp_state == GP_IDLE) {
 128                 WRITE_ONCE(rsp->gp_state, GP_ENTER);
 129                 WARN_ON_ONCE(rsp->gp_count);
 130                 /*
 131                  * Note that we could simply do rcu_sync_call(rsp) here and
 132                  * avoid the "if (gp_state == GP_IDLE)" block below.
 133                  *
 134                  * However, synchronize_rcu() can be faster if rcu_expedited
 135                  * or rcu_blocking_is_gp() is true.
 136                  *
 137                  * Another reason is that we can't wait for rcu callback if
 138                  * we are called at early boot time but this shouldn't happen.
 139                  */
 140         }
 141         rsp->gp_count++;
 142         spin_unlock_irq(&rsp->rss_lock);
 143
 144         if (gp_state == GP_IDLE) {
 145                 /*
 146                  * See the comment above, this simply does the "synchronous"
 147                  * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED.
 148                  */
 149                 synchronize_rcu();
 150                 rcu_sync_func(&rsp->cb_head);
 151                 /* Not really needed, wait_event() would see GP_PASSED. */
 152                 return;
 153         }
 154
 155         wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED);
 156 }
 157
 158 /**
 159  * rcu_sync_exit() - Allow readers back onto fast path after grace period
 160  * @rsp: Pointer to rcu_sync structure to use for synchronization
 161  *
 162  * This function is used by updaters who have completed, and can therefore
 163  * now allow readers to make use of their fastpaths after a grace period
 164  * has elapsed.  After this grace period has completed, all subsequent
 165  * calls to rcu_sync_is_idle() will return true, which tells readers that
 166  * they can once again use their fastpaths.
 167  */
 168 void rcu_sync_exit(struct rcu_sync *rsp)
 169 {
 170         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
 171         WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
 172
 173         spin_lock_irq(&rsp->rss_lock);
 174         if (!--rsp->gp_count) {
 175                 if (rsp->gp_state == GP_PASSED) {
 176                         WRITE_ONCE(rsp->gp_state, GP_EXIT);
 177                         rcu_sync_call(rsp);
 178                 } else if (rsp->gp_state == GP_EXIT) {
 179                         WRITE_ONCE(rsp->gp_state, GP_REPLAY);
 180                 }
 181         }
 182         spin_unlock_irq(&rsp->rss_lock);
 183 }
 184
 185 /**
 186  * rcu_sync_dtor() - Clean up an rcu_sync structure
 187  * @rsp: Pointer to rcu_sync structure to be cleaned up
 188  */
 189 void rcu_sync_dtor(struct rcu_sync *rsp)
 190 {
 191         int gp_state;
 192
 193         WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
 194         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
 195
 196         spin_lock_irq(&rsp->rss_lock);
 197         if (rsp->gp_state == GP_REPLAY)
 198                 WRITE_ONCE(rsp->gp_state, GP_EXIT);
 199         gp_state = rsp->gp_state;
 200         spin_unlock_irq(&rsp->rss_lock);
 201
 202         if (gp_state != GP_IDLE) {
 203                 rcu_barrier();
 204                 WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
 205         }
 206 }