// SPDX-License-Identifier: GPL-2.0+
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/*
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* RCU-based infrastructure for lightweight reader-writer locking
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*
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* Copyright (c) 2015, Red Hat, Inc.
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*
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* Author: Oleg Nesterov <oleg@redhat.com>
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*/
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#include <linux/rcu_sync.h>
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#include <linux/sched.h>
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enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY };
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#define rss_lock gp_wait.lock
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/**
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* rcu_sync_init() - Initialize an rcu_sync structure
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* @rsp: Pointer to rcu_sync structure to be initialized
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*/
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void rcu_sync_init(struct rcu_sync *rsp)
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{
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memset(rsp, 0, sizeof(*rsp));
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init_waitqueue_head(&rsp->gp_wait);
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}
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/**
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* rcu_sync_enter_start - Force readers onto slow path for multiple updates
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* Must be called after rcu_sync_init() and before first use.
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*
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* Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
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* pairs turn into NO-OPs.
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*/
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void rcu_sync_enter_start(struct rcu_sync *rsp)
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{
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rsp->gp_count++;
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rsp->gp_state = GP_PASSED;
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}
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static void rcu_sync_func(struct rcu_head *rhp);
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static void rcu_sync_call(struct rcu_sync *rsp)
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{
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call_rcu(&rsp->cb_head, rcu_sync_func);
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}
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/**
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* rcu_sync_func() - Callback function managing reader access to fastpath
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* @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
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*
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* This function is passed to call_rcu() function by rcu_sync_enter() and
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* rcu_sync_exit(), so that it is invoked after a grace period following the
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* that invocation of enter/exit.
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*
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* If it is called by rcu_sync_enter() it signals that all the readers were
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* switched onto slow path.
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*
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* If it is called by rcu_sync_exit() it takes action based on events that
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* have taken place in the meantime, so that closely spaced rcu_sync_enter()
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* and rcu_sync_exit() pairs need not wait for a grace period.
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*
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* If another rcu_sync_enter() is invoked before the grace period
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* ended, reset state to allow the next rcu_sync_exit() to let the
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* readers back onto their fastpaths (after a grace period). If both
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* another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
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* before the grace period ended, re-invoke call_rcu() on behalf of that
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* rcu_sync_exit(). Otherwise, set all state back to idle so that readers
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* can again use their fastpaths.
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*/
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static void rcu_sync_func(struct rcu_head *rhp)
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{
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struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
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unsigned long flags;
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
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spin_lock_irqsave(&rsp->rss_lock, flags);
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if (rsp->gp_count) {
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/*
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* We're at least a GP after the GP_IDLE->GP_ENTER transition.
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*/
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WRITE_ONCE(rsp->gp_state, GP_PASSED);
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wake_up_locked(&rsp->gp_wait);
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} else if (rsp->gp_state == GP_REPLAY) {
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/*
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* A new rcu_sync_exit() has happened; requeue the callback to
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* catch a later GP.
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*/
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WRITE_ONCE(rsp->gp_state, GP_EXIT);
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rcu_sync_call(rsp);
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} else {
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/*
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* We're at least a GP after the last rcu_sync_exit(); eveybody
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* will now have observed the write side critical section.
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* Let 'em rip!.
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*/
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WRITE_ONCE(rsp->gp_state, GP_IDLE);
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}
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spin_unlock_irqrestore(&rsp->rss_lock, flags);
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}
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/**
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* rcu_sync_enter() - Force readers onto slowpath
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* This function is used by updaters who need readers to make use of
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* a slowpath during the update. After this function returns, all
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* subsequent calls to rcu_sync_is_idle() will return false, which
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* tells readers to stay off their fastpaths. A later call to
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* rcu_sync_exit() re-enables reader slowpaths.
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*
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* When called in isolation, rcu_sync_enter() must wait for a grace
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* period, however, closely spaced calls to rcu_sync_enter() can
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* optimize away the grace-period wait via a state machine implemented
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* by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
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*/
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void rcu_sync_enter(struct rcu_sync *rsp)
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{
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int gp_state;
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spin_lock_irq(&rsp->rss_lock);
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gp_state = rsp->gp_state;
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if (gp_state == GP_IDLE) {
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WRITE_ONCE(rsp->gp_state, GP_ENTER);
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WARN_ON_ONCE(rsp->gp_count);
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/*
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* Note that we could simply do rcu_sync_call(rsp) here and
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* avoid the "if (gp_state == GP_IDLE)" block below.
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*
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* However, synchronize_rcu() can be faster if rcu_expedited
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* or rcu_blocking_is_gp() is true.
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*
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* Another reason is that we can't wait for rcu callback if
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* we are called at early boot time but this shouldn't happen.
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*/
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}
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rsp->gp_count++;
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spin_unlock_irq(&rsp->rss_lock);
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if (gp_state == GP_IDLE) {
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/*
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* See the comment above, this simply does the "synchronous"
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* call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED.
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*/
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synchronize_rcu();
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rcu_sync_func(&rsp->cb_head);
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/* Not really needed, wait_event() would see GP_PASSED. */
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return;
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}
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wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED);
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}
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/**
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* rcu_sync_exit() - Allow readers back onto fast path after grace period
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* This function is used by updaters who have completed, and can therefore
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* now allow readers to make use of their fastpaths after a grace period
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* has elapsed. After this grace period has completed, all subsequent
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* calls to rcu_sync_is_idle() will return true, which tells readers that
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* they can once again use their fastpaths.
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*/
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void rcu_sync_exit(struct rcu_sync *rsp)
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{
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
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WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
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spin_lock_irq(&rsp->rss_lock);
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if (!--rsp->gp_count) {
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if (rsp->gp_state == GP_PASSED) {
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WRITE_ONCE(rsp->gp_state, GP_EXIT);
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rcu_sync_call(rsp);
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} else if (rsp->gp_state == GP_EXIT) {
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WRITE_ONCE(rsp->gp_state, GP_REPLAY);
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}
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}
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spin_unlock_irq(&rsp->rss_lock);
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}
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/**
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* rcu_sync_dtor() - Clean up an rcu_sync structure
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* @rsp: Pointer to rcu_sync structure to be cleaned up
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*/
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void rcu_sync_dtor(struct rcu_sync *rsp)
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{
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int gp_state;
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WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
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spin_lock_irq(&rsp->rss_lock);
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if (rsp->gp_state == GP_REPLAY)
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WRITE_ONCE(rsp->gp_state, GP_EXIT);
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gp_state = rsp->gp_state;
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spin_unlock_irq(&rsp->rss_lock);
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if (gp_state != GP_IDLE) {
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rcu_barrier();
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WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
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}
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}
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