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 #ifdef CONFIG_PROVE_RCU
  14 #define __INIT_HELD(func)       .held = func,
  15 #else
  16 #define __INIT_HELD(func)
  17 #endif
  18
  19 static const struct {
  20         void (*sync)(void);
  21         void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
  22         void (*wait)(void);
  23 #ifdef CONFIG_PROVE_RCU
  24         int  (*held)(void);
  25 #endif
  26 } gp_ops[] = {
  27         [RCU_SYNC] = {
  28                 .sync = synchronize_rcu,
  29                 .call = call_rcu,
  30                 .wait = rcu_barrier,
  31                 __INIT_HELD(rcu_read_lock_held)
  32         },
  33         [RCU_SCHED_SYNC] = {
  34                 .sync = synchronize_rcu,
  35                 .call = call_rcu,
  36                 .wait = rcu_barrier,
  37                 __INIT_HELD(rcu_read_lock_sched_held)
  38         },
  39         [RCU_BH_SYNC] = {
  40                 .sync = synchronize_rcu,
  41                 .call = call_rcu,
  42                 .wait = rcu_barrier,
  43                 __INIT_HELD(rcu_read_lock_bh_held)
  44         },
  45 };
  46
  47 enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
  48 enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
  49
  50 #define rss_lock        gp_wait.lock
  51
  52 #ifdef CONFIG_PROVE_RCU
  53 void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
  54 {
  55         RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
  56                          "suspicious rcu_sync_is_idle() usage");
  57 }
  58
  59 EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
  60 #endif
  61
  62 /**
  63  * rcu_sync_init() - Initialize an rcu_sync structure
  64  * @rsp: Pointer to rcu_sync structure to be initialized
  65  * @type: Flavor of RCU with which to synchronize rcu_sync structure
  66  */
  67 void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
  68 {
  69         memset(rsp, 0, sizeof(*rsp));
  70         init_waitqueue_head(&rsp->gp_wait);
  71         rsp->gp_type = type;
  72 }
  73
  74 /**
  75  * rcu_sync_enter_start - Force readers onto slow path for multiple updates
  76  * @rsp: Pointer to rcu_sync structure to use for synchronization
  77  *
  78  * Must be called after rcu_sync_init() and before first use.
  79  *
  80  * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
  81  * pairs turn into NO-OPs.
  82  */
  83 void rcu_sync_enter_start(struct rcu_sync *rsp)
  84 {
  85         rsp->gp_count++;
  86         rsp->gp_state = GP_PASSED;
  87 }
  88
  89 /**
  90  * rcu_sync_enter() - Force readers onto slowpath
  91  * @rsp: Pointer to rcu_sync structure to use for synchronization
  92  *
  93  * This function is used by updaters who need readers to make use of
  94  * a slowpath during the update.  After this function returns, all
  95  * subsequent calls to rcu_sync_is_idle() will return false, which
  96  * tells readers to stay off their fastpaths.  A later call to
  97  * rcu_sync_exit() re-enables reader slowpaths.
  98  *
  99  * When called in isolation, rcu_sync_enter() must wait for a grace
 100  * period, however, closely spaced calls to rcu_sync_enter() can
 101  * optimize away the grace-period wait via a state machine implemented
 102  * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
 103  */
 104 void rcu_sync_enter(struct rcu_sync *rsp)
 105 {
 106         bool need_wait, need_sync;
 107
 108         spin_lock_irq(&rsp->rss_lock);
 109         need_wait = rsp->gp_count++;
 110         need_sync = rsp->gp_state == GP_IDLE;
 111         if (need_sync)
 112                 rsp->gp_state = GP_PENDING;
 113         spin_unlock_irq(&rsp->rss_lock);
 114
 115         WARN_ON_ONCE(need_wait && need_sync);
 116         if (need_sync) {
 117                 gp_ops[rsp->gp_type].sync();
 118                 rsp->gp_state = GP_PASSED;
 119                 wake_up_all(&rsp->gp_wait);
 120         } else if (need_wait) {
 121                 wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
 122         } else {
 123                 /*
 124                  * Possible when there's a pending CB from a rcu_sync_exit().
 125                  * Nobody has yet been allowed the 'fast' path and thus we can
 126                  * avoid doing any sync(). The callback will get 'dropped'.
 127                  */
 128                 WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
 129         }
 130 }
 131
 132 /**
 133  * rcu_sync_func() - Callback function managing reader access to fastpath
 134  * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
 135  *
 136  * This function is passed to one of the call_rcu() functions by
 137  * rcu_sync_exit(), so that it is invoked after a grace period following the
 138  * that invocation of rcu_sync_exit().  It takes action based on events that
 139  * have taken place in the meantime, so that closely spaced rcu_sync_enter()
 140  * and rcu_sync_exit() pairs need not wait for a grace period.
 141  *
 142  * If another rcu_sync_enter() is invoked before the grace period
 143  * ended, reset state to allow the next rcu_sync_exit() to let the
 144  * readers back onto their fastpaths (after a grace period).  If both
 145  * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
 146  * before the grace period ended, re-invoke call_rcu() on behalf of that
 147  * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
 148  * can again use their fastpaths.
 149  */
 150 static void rcu_sync_func(struct rcu_head *rhp)
 151 {
 152         struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
 153         unsigned long flags;
 154
 155         WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
 156         WARN_ON_ONCE(rsp->cb_state == CB_IDLE);
 157
 158         spin_lock_irqsave(&rsp->rss_lock, flags);
 159         if (rsp->gp_count) {
 160                 /*
 161                  * A new rcu_sync_begin() has happened; drop the callback.
 162                  */
 163                 rsp->cb_state = CB_IDLE;
 164         } else if (rsp->cb_state == CB_REPLAY) {
 165                 /*
 166                  * A new rcu_sync_exit() has happened; requeue the callback
 167                  * to catch a later GP.
 168                  */
 169                 rsp->cb_state = CB_PENDING;
 170                 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
 171         } else {
 172                 /*
 173                  * We're at least a GP after rcu_sync_exit(); eveybody will now
 174                  * have observed the write side critical section. Let 'em rip!.
 175                  */
 176                 rsp->cb_state = CB_IDLE;
 177                 rsp->gp_state = GP_IDLE;
 178         }
 179         spin_unlock_irqrestore(&rsp->rss_lock, flags);
 180 }
 181
 182 /**
 183  * rcu_sync_exit() - Allow readers back onto fast patch after grace period
 184  * @rsp: Pointer to rcu_sync structure to use for synchronization
 185  *
 186  * This function is used by updaters who have completed, and can therefore
 187  * now allow readers to make use of their fastpaths after a grace period
 188  * has elapsed.  After this grace period has completed, all subsequent
 189  * calls to rcu_sync_is_idle() will return true, which tells readers that
 190  * they can once again use their fastpaths.
 191  */
 192 void rcu_sync_exit(struct rcu_sync *rsp)
 193 {
 194         spin_lock_irq(&rsp->rss_lock);
 195         if (!--rsp->gp_count) {
 196                 if (rsp->cb_state == CB_IDLE) {
 197                         rsp->cb_state = CB_PENDING;
 198                         gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
 199                 } else if (rsp->cb_state == CB_PENDING) {
 200                         rsp->cb_state = CB_REPLAY;
 201                 }
 202         }
 203         spin_unlock_irq(&rsp->rss_lock);
 204 }
 205
 206 /**
 207  * rcu_sync_dtor() - Clean up an rcu_sync structure
 208  * @rsp: Pointer to rcu_sync structure to be cleaned up
 209  */
 210 void rcu_sync_dtor(struct rcu_sync *rsp)
 211 {
 212         int cb_state;
 213
 214         WARN_ON_ONCE(rsp->gp_count);
 215
 216         spin_lock_irq(&rsp->rss_lock);
 217         if (rsp->cb_state == CB_REPLAY)
 218                 rsp->cb_state = CB_PENDING;
 219         cb_state = rsp->cb_state;
 220         spin_unlock_irq(&rsp->rss_lock);
 221
 222         if (cb_state != CB_IDLE) {
 223                 gp_ops[rsp->gp_type].wait();
 224                 WARN_ON_ONCE(rsp->cb_state != CB_IDLE);
 225         }
 226 }