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 static void rcu_sync_func(struct rcu_head *rhp);
  28
  29 static void rcu_sync_call(struct rcu_sync *rsp)
  30 {
  31         call_rcu_hurry(&rsp->cb_head, rcu_sync_func);
  32 }
  33
  34 /**
  35  * rcu_sync_func() - Callback function managing reader access to fastpath
  36  * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
  37  *
  38  * This function is passed to call_rcu() function by rcu_sync_enter() and
  39  * rcu_sync_exit(), so that it is invoked after a grace period following the
  40  * that invocation of enter/exit.
  41  *
  42  * If it is called by rcu_sync_enter() it signals that all the readers were
  43  * switched onto slow path.
  44  *
  45  * If it is called by rcu_sync_exit() it takes action based on events that
  46  * have taken place in the meantime, so that closely spaced rcu_sync_enter()
  47  * and rcu_sync_exit() pairs need not wait for a grace period.
  48  *
  49  * If another rcu_sync_enter() is invoked before the grace period
  50  * ended, reset state to allow the next rcu_sync_exit() to let the
  51  * readers back onto their fastpaths (after a grace period).  If both
  52  * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
  53  * before the grace period ended, re-invoke call_rcu() on behalf of that
  54  * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
  55  * can again use their fastpaths.
  56  */
  57 static void rcu_sync_func(struct rcu_head *rhp)
  58 {
  59         struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
  60         unsigned long flags;
  61
  62         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
  63         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
  64
  65         spin_lock_irqsave(&rsp->rss_lock, flags);
  66         if (rsp->gp_count) {
  67                 /*
  68                  * We're at least a GP after the GP_IDLE->GP_ENTER transition.
  69                  */
  70                 WRITE_ONCE(rsp->gp_state, GP_PASSED);
  71                 wake_up_locked(&rsp->gp_wait);
  72         } else if (rsp->gp_state == GP_REPLAY) {
  73                 /*
  74                  * A new rcu_sync_exit() has happened; requeue the callback to
  75                  * catch a later GP.
  76                  */
  77                 WRITE_ONCE(rsp->gp_state, GP_EXIT);
  78                 rcu_sync_call(rsp);
  79         } else {
  80                 /*
  81                  * We're at least a GP after the last rcu_sync_exit(); everybody
  82                  * will now have observed the write side critical section.
  83                  * Let 'em rip!
  84                  */
  85                 WRITE_ONCE(rsp->gp_state, GP_IDLE);
  86         }
  87         spin_unlock_irqrestore(&rsp->rss_lock, flags);
  88 }
  89
  90 /**
  91  * rcu_sync_enter() - Force readers onto slowpath
  92  * @rsp: Pointer to rcu_sync structure to use for synchronization
  93  *
  94  * This function is used by updaters who need readers to make use of
  95  * a slowpath during the update.  After this function returns, all
  96  * subsequent calls to rcu_sync_is_idle() will return false, which
  97  * tells readers to stay off their fastpaths.  A later call to
  98  * rcu_sync_exit() re-enables reader fastpaths.
  99  *
 100  * When called in isolation, rcu_sync_enter() must wait for a grace
 101  * period, however, closely spaced calls to rcu_sync_enter() can
 102  * optimize away the grace-period wait via a state machine implemented
 103  * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
 104  */
 105 void rcu_sync_enter(struct rcu_sync *rsp)
 106 {
 107         int gp_state;
 108
 109         spin_lock_irq(&rsp->rss_lock);
 110         gp_state = rsp->gp_state;
 111         if (gp_state == GP_IDLE) {
 112                 WRITE_ONCE(rsp->gp_state, GP_ENTER);
 113                 WARN_ON_ONCE(rsp->gp_count);
 114                 /*
 115                  * Note that we could simply do rcu_sync_call(rsp) here and
 116                  * avoid the "if (gp_state == GP_IDLE)" block below.
 117                  *
 118                  * However, synchronize_rcu() can be faster if rcu_expedited
 119                  * or rcu_blocking_is_gp() is true.
 120                  *
 121                  * Another reason is that we can't wait for rcu callback if
 122                  * we are called at early boot time but this shouldn't happen.
 123                  */
 124         }
 125         rsp->gp_count++;
 126         spin_unlock_irq(&rsp->rss_lock);
 127
 128         if (gp_state == GP_IDLE) {
 129                 /*
 130                  * See the comment above, this simply does the "synchronous"
 131                  * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED.
 132                  */
 133                 synchronize_rcu();
 134                 rcu_sync_func(&rsp->cb_head);
 135                 /* Not really needed, wait_event() would see GP_PASSED. */
 136                 return;
 137         }
 138
 139         wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED);
 140 }
 141
 142 /**
 143  * rcu_sync_exit() - Allow readers back onto fast path after grace period
 144  * @rsp: Pointer to rcu_sync structure to use for synchronization
 145  *
 146  * This function is used by updaters who have completed, and can therefore
 147  * now allow readers to make use of their fastpaths after a grace period
 148  * has elapsed.  After this grace period has completed, all subsequent
 149  * calls to rcu_sync_is_idle() will return true, which tells readers that
 150  * they can once again use their fastpaths.
 151  */
 152 void rcu_sync_exit(struct rcu_sync *rsp)
 153 {
 154         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
 155
 156         spin_lock_irq(&rsp->rss_lock);
 157         WARN_ON_ONCE(rsp->gp_count == 0);
 158         if (!--rsp->gp_count) {
 159                 if (rsp->gp_state == GP_PASSED) {
 160                         WRITE_ONCE(rsp->gp_state, GP_EXIT);
 161                         rcu_sync_call(rsp);
 162                 } else if (rsp->gp_state == GP_EXIT) {
 163                         WRITE_ONCE(rsp->gp_state, GP_REPLAY);
 164                 }
 165         }
 166         spin_unlock_irq(&rsp->rss_lock);
 167 }
 168
 169 /**
 170  * rcu_sync_dtor() - Clean up an rcu_sync structure
 171  * @rsp: Pointer to rcu_sync structure to be cleaned up
 172  */
 173 void rcu_sync_dtor(struct rcu_sync *rsp)
 174 {
 175         int gp_state;
 176
 177         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
 178
 179         spin_lock_irq(&rsp->rss_lock);
 180         WARN_ON_ONCE(rsp->gp_count);
 181         if (rsp->gp_state == GP_REPLAY)
 182                 WRITE_ONCE(rsp->gp_state, GP_EXIT);
 183         gp_state = rsp->gp_state;
 184         spin_unlock_irq(&rsp->rss_lock);
 185
 186         if (gp_state != GP_IDLE) {
 187                 rcu_barrier();
 188                 WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
 189         }
 190 }