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       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                 :          3 : void rcu_sync_init(struct rcu_sync *rsp)
      22                 :            : {
      23                 :          3 :         memset(rsp, 0, sizeof(*rsp));
      24                 :          3 :         init_waitqueue_head(&rsp->gp_wait);
      25                 :          3 : }
      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                 :          3 : void rcu_sync_enter_start(struct rcu_sync *rsp)
      37                 :            : {
      38                 :          3 :         rsp->gp_count++;
      39                 :          3 :         rsp->gp_state = GP_PASSED;
      40                 :          3 : }
      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                 :          3 :         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                 :          3 : 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                 :          3 :         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
      79                 :          3 :         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
      80                 :            : 
      81                 :          3 :         spin_lock_irqsave(&rsp->rss_lock, flags);
      82                 :          3 :         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                 :          3 :                 wake_up_locked(&rsp->gp_wait);
      88                 :          3 :         } 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                 :          3 : }
     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                 :          3 : void rcu_sync_enter(struct rcu_sync *rsp)
     122                 :            : {
     123                 :            :         int gp_state;
     124                 :            : 
     125                 :            :         spin_lock_irq(&rsp->rss_lock);
     126                 :          3 :         gp_state = rsp->gp_state;
     127                 :          3 :         if (gp_state == GP_IDLE) {
     128                 :            :                 WRITE_ONCE(rsp->gp_state, GP_ENTER);
     129                 :          3 :                 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                 :          3 :         rsp->gp_count++;
     142                 :            :         spin_unlock_irq(&rsp->rss_lock);
     143                 :            : 
     144                 :          3 :         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                 :          3 :                 synchronize_rcu();
     150                 :          3 :                 rcu_sync_func(&rsp->cb_head);
     151                 :            :                 /* Not really needed, wait_event() would see GP_PASSED. */
     152                 :          3 :                 return;
     153                 :            :         }
     154                 :            : 
     155                 :          3 :         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                 :          3 : void rcu_sync_exit(struct rcu_sync *rsp)
     169                 :            : {
     170                 :          3 :         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
     171                 :          3 :         WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
     172                 :            : 
     173                 :            :         spin_lock_irq(&rsp->rss_lock);
     174                 :          3 :         if (!--rsp->gp_count) {
     175                 :          3 :                 if (rsp->gp_state == GP_PASSED) {
     176                 :            :                         WRITE_ONCE(rsp->gp_state, GP_EXIT);
     177                 :            :                         rcu_sync_call(rsp);
     178                 :          0 :                 } 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                 :          3 : }
     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                 :          3 : void rcu_sync_dtor(struct rcu_sync *rsp)
     190                 :            : {
     191                 :            :         int gp_state;
     192                 :            : 
     193                 :          3 :         WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
     194                 :          3 :         WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
     195                 :            : 
     196                 :            :         spin_lock_irq(&rsp->rss_lock);
     197                 :          3 :         if (rsp->gp_state == GP_REPLAY)
     198                 :            :                 WRITE_ONCE(rsp->gp_state, GP_EXIT);
     199                 :          3 :         gp_state = rsp->gp_state;
     200                 :            :         spin_unlock_irq(&rsp->rss_lock);
     201                 :            : 
     202                 :          3 :         if (gp_state != GP_IDLE) {
     203                 :          0 :                 rcu_barrier();
     204                 :          0 :                 WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
     205                 :            :         }
     206                 :          3 : }