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
 13enum { 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 */
 21void rcu_sync_init(struct rcu_sync *rsp)
 22{
 23	memset(rsp, 0, sizeof(*rsp));
 24	init_waitqueue_head(&rsp->gp_wait);
 25}
 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 */
 36void rcu_sync_enter_start(struct rcu_sync *rsp)
 37{
 38	rsp->gp_count++;
 39	rsp->gp_state = GP_PASSED;
 40}
 41
 42
 43static void rcu_sync_func(struct rcu_head *rhp);
 44
 45static void rcu_sync_call(struct rcu_sync *rsp)
 46{
 47	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 */
 73static 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	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
 79	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
 80
 81	spin_lock_irqsave(&rsp->rss_lock, flags);
 82	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		wake_up_locked(&rsp->gp_wait);
 88	} 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}
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 */
121void rcu_sync_enter(struct rcu_sync *rsp)
122{
123	int gp_state;
124
125	spin_lock_irq(&rsp->rss_lock);
126	gp_state = rsp->gp_state;
127	if (gp_state == GP_IDLE) {
128		WRITE_ONCE(rsp->gp_state, GP_ENTER);
129		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	rsp->gp_count++;
142	spin_unlock_irq(&rsp->rss_lock);
143
144	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		synchronize_rcu();
150		rcu_sync_func(&rsp->cb_head);
151		/* Not really needed, wait_event() would see GP_PASSED. */
152		return;
153	}
154
155	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 */
168void rcu_sync_exit(struct rcu_sync *rsp)
169{
170	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
171	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
172
173	spin_lock_irq(&rsp->rss_lock);
174	if (!--rsp->gp_count) {
175		if (rsp->gp_state == GP_PASSED) {
176			WRITE_ONCE(rsp->gp_state, GP_EXIT);
177			rcu_sync_call(rsp);
178		} 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}
184
185/**
186 * rcu_sync_dtor() - Clean up an rcu_sync structure
187 * @rsp: Pointer to rcu_sync structure to be cleaned up
188 */
189void rcu_sync_dtor(struct rcu_sync *rsp)
190{
191	int gp_state;
192
193	WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
194	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
195
196	spin_lock_irq(&rsp->rss_lock);
197	if (rsp->gp_state == GP_REPLAY)
198		WRITE_ONCE(rsp->gp_state, GP_EXIT);
199	gp_state = rsp->gp_state;
200	spin_unlock_irq(&rsp->rss_lock);
201
202	if (gp_state != GP_IDLE) {
203		rcu_barrier();
204		WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
205	}
206}