kernel/rcu/sync.c

0001 // SPDX-License-Identifier: GPL-2.0+
0002 /*
0003  * RCU-based infrastructure for lightweight reader-writer locking
0004  *
0005  * Copyright (c) 2015, Red Hat, Inc.
0006  *
0007  * Author: Oleg Nesterov <oleg@redhat.com>
0008  */
0009
0010 #include <linux/rcu_sync.h>
0011 #include <linux/sched.h>
0012
0013 enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY };
0014
0015 #define rss_lock    gp_wait.lock
0016
0017 /**
0018  * rcu_sync_init() - Initialize an rcu_sync structure
0019  * @rsp: Pointer to rcu_sync structure to be initialized
0020  */
0021 void rcu_sync_init(struct rcu_sync *rsp)
0022 {
0023     memset(rsp, 0, sizeof(*rsp));
0024     init_waitqueue_head(&rsp->gp_wait);
0025 }
0026
0027 /**
0028  * rcu_sync_enter_start - Force readers onto slow path for multiple updates
0029  * @rsp: Pointer to rcu_sync structure to use for synchronization
0030  *
0031  * Must be called after rcu_sync_init() and before first use.
0032  *
0033  * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
0034  * pairs turn into NO-OPs.
0035  */
0036 void rcu_sync_enter_start(struct rcu_sync *rsp)
0037 {
0038     rsp->gp_count++;
0039     rsp->gp_state = GP_PASSED;
0040 }
0041
0042
0043 static void rcu_sync_func(struct rcu_head *rhp);
0044
0045 static void rcu_sync_call(struct rcu_sync *rsp)
0046 {
0047     call_rcu(&rsp->cb_head, rcu_sync_func);
0048 }
0049
0050 /**
0051  * rcu_sync_func() - Callback function managing reader access to fastpath
0052  * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
0053  *
0054  * This function is passed to call_rcu() function by rcu_sync_enter() and
0055  * rcu_sync_exit(), so that it is invoked after a grace period following the
0056  * that invocation of enter/exit.
0057  *
0058  * If it is called by rcu_sync_enter() it signals that all the readers were
0059  * switched onto slow path.
0060  *
0061  * If it is called by rcu_sync_exit() it takes action based on events that
0062  * have taken place in the meantime, so that closely spaced rcu_sync_enter()
0063  * and rcu_sync_exit() pairs need not wait for a grace period.
0064  *
0065  * If another rcu_sync_enter() is invoked before the grace period
0066  * ended, reset state to allow the next rcu_sync_exit() to let the
0067  * readers back onto their fastpaths (after a grace period).  If both
0068  * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
0069  * before the grace period ended, re-invoke call_rcu() on behalf of that
0070  * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
0071  * can again use their fastpaths.
0072  */
0073 static void rcu_sync_func(struct rcu_head *rhp)
0074 {
0075     struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
0076     unsigned long flags;
0077
0078     WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
0079     WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
0080
0081     spin_lock_irqsave(&rsp->rss_lock, flags);
0082     if (rsp->gp_count) {
0083         /*
0084          * We're at least a GP after the GP_IDLE->GP_ENTER transition.
0085          */
0086         WRITE_ONCE(rsp->gp_state, GP_PASSED);
0087         wake_up_locked(&rsp->gp_wait);
0088     } else if (rsp->gp_state == GP_REPLAY) {
0089         /*
0090          * A new rcu_sync_exit() has happened; requeue the callback to
0091          * catch a later GP.
0092          */
0093         WRITE_ONCE(rsp->gp_state, GP_EXIT);
0094         rcu_sync_call(rsp);
0095     } else {
0096         /*
0097          * We're at least a GP after the last rcu_sync_exit(); everybody
0098          * will now have observed the write side critical section.
0099          * Let 'em rip!
0100          */
0101         WRITE_ONCE(rsp->gp_state, GP_IDLE);
0102     }
0103     spin_unlock_irqrestore(&rsp->rss_lock, flags);
0104 }
0105
0106 /**
0107  * rcu_sync_enter() - Force readers onto slowpath
0108  * @rsp: Pointer to rcu_sync structure to use for synchronization
0109  *
0110  * This function is used by updaters who need readers to make use of
0111  * a slowpath during the update.  After this function returns, all
0112  * subsequent calls to rcu_sync_is_idle() will return false, which
0113  * tells readers to stay off their fastpaths.  A later call to
0114  * rcu_sync_exit() re-enables reader fastpaths.
0115  *
0116  * When called in isolation, rcu_sync_enter() must wait for a grace
0117  * period, however, closely spaced calls to rcu_sync_enter() can
0118  * optimize away the grace-period wait via a state machine implemented
0119  * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
0120  */
0121 void rcu_sync_enter(struct rcu_sync *rsp)
0122 {
0123     int gp_state;
0124
0125     spin_lock_irq(&rsp->rss_lock);
0126     gp_state = rsp->gp_state;
0127     if (gp_state == GP_IDLE) {
0128         WRITE_ONCE(rsp->gp_state, GP_ENTER);
0129         WARN_ON_ONCE(rsp->gp_count);
0130         /*
0131          * Note that we could simply do rcu_sync_call(rsp) here and
0132          * avoid the "if (gp_state == GP_IDLE)" block below.
0133          *
0134          * However, synchronize_rcu() can be faster if rcu_expedited
0135          * or rcu_blocking_is_gp() is true.
0136          *
0137          * Another reason is that we can't wait for rcu callback if
0138          * we are called at early boot time but this shouldn't happen.
0139          */
0140     }
0141     rsp->gp_count++;
0142     spin_unlock_irq(&rsp->rss_lock);
0143
0144     if (gp_state == GP_IDLE) {
0145         /*
0146          * See the comment above, this simply does the "synchronous"
0147          * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED.
0148          */
0149         synchronize_rcu();
0150         rcu_sync_func(&rsp->cb_head);
0151         /* Not really needed, wait_event() would see GP_PASSED. */
0152         return;
0153     }
0154
0155     wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED);
0156 }
0157
0158 /**
0159  * rcu_sync_exit() - Allow readers back onto fast path after grace period
0160  * @rsp: Pointer to rcu_sync structure to use for synchronization
0161  *
0162  * This function is used by updaters who have completed, and can therefore
0163  * now allow readers to make use of their fastpaths after a grace period
0164  * has elapsed.  After this grace period has completed, all subsequent
0165  * calls to rcu_sync_is_idle() will return true, which tells readers that
0166  * they can once again use their fastpaths.
0167  */
0168 void rcu_sync_exit(struct rcu_sync *rsp)
0169 {
0170     WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
0171     WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
0172
0173     spin_lock_irq(&rsp->rss_lock);
0174     if (!--rsp->gp_count) {
0175         if (rsp->gp_state == GP_PASSED) {
0176             WRITE_ONCE(rsp->gp_state, GP_EXIT);
0177             rcu_sync_call(rsp);
0178         } else if (rsp->gp_state == GP_EXIT) {
0179             WRITE_ONCE(rsp->gp_state, GP_REPLAY);
0180         }
0181     }
0182     spin_unlock_irq(&rsp->rss_lock);
0183 }
0184
0185 /**
0186  * rcu_sync_dtor() - Clean up an rcu_sync structure
0187  * @rsp: Pointer to rcu_sync structure to be cleaned up
0188  */
0189 void rcu_sync_dtor(struct rcu_sync *rsp)
0190 {
0191     int gp_state;
0192
0193     WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
0194     WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
0195
0196     spin_lock_irq(&rsp->rss_lock);
0197     if (rsp->gp_state == GP_REPLAY)
0198         WRITE_ONCE(rsp->gp_state, GP_EXIT);
0199     gp_state = rsp->gp_state;
0200     spin_unlock_irq(&rsp->rss_lock);
0201
0202     if (gp_state != GP_IDLE) {
0203         rcu_barrier();
0204         WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
0205     }
0206 }