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
 27static void rcu_sync_func(struct rcu_head *rhp);
 28
 29static void rcu_sync_call(struct rcu_sync *rsp)
 30{
 31	call_rcu_hurry(&rsp->cb_head, rcu_sync_func);
 32}
 33
 34/**
 35 * rcu_sync_func() - Callback function managing reader access to fastpath
 36 * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
 37 *
 38 * This function is passed to call_rcu() function by rcu_sync_enter() and
 39 * rcu_sync_exit(), so that it is invoked after a grace period following the
 40 * that invocation of enter/exit.
 41 *
 42 * If it is called by rcu_sync_enter() it signals that all the readers were
 43 * switched onto slow path.
 44 *
 45 * If it is called by rcu_sync_exit() it takes action based on events that
 46 * have taken place in the meantime, so that closely spaced rcu_sync_enter()
 47 * and rcu_sync_exit() pairs need not wait for a grace period.
 48 *
 49 * If another rcu_sync_enter() is invoked before the grace period
 50 * ended, reset state to allow the next rcu_sync_exit() to let the
 51 * readers back onto their fastpaths (after a grace period).  If both
 52 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
 53 * before the grace period ended, re-invoke call_rcu() on behalf of that
 54 * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
 55 * can again use their fastpaths.
 56 */
 57static void rcu_sync_func(struct rcu_head *rhp)
 58{
 59	struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
 60	unsigned long flags;
 61
 62	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
 63	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
 64
 65	spin_lock_irqsave(&rsp->rss_lock, flags);
 66	if (rsp->gp_count) {
 67		/*
 68		 * We're at least a GP after the GP_IDLE->GP_ENTER transition.
 69		 */
 70		WRITE_ONCE(rsp->gp_state, GP_PASSED);
 71		wake_up_locked(&rsp->gp_wait);
 72	} else if (rsp->gp_state == GP_REPLAY) {
 73		/*
 74		 * A new rcu_sync_exit() has happened; requeue the callback to
 75		 * catch a later GP.
 76		 */
 77		WRITE_ONCE(rsp->gp_state, GP_EXIT);
 78		rcu_sync_call(rsp);
 79	} else {
 80		/*
 81		 * We're at least a GP after the last rcu_sync_exit(); everybody
 82		 * will now have observed the write side critical section.
 83		 * Let 'em rip!
 84		 */
 85		WRITE_ONCE(rsp->gp_state, GP_IDLE);
 86	}
 87	spin_unlock_irqrestore(&rsp->rss_lock, flags);
 88}
 89
 90/**
 91 * rcu_sync_enter() - Force readers onto slowpath
 92 * @rsp: Pointer to rcu_sync structure to use for synchronization
 93 *
 94 * This function is used by updaters who need readers to make use of
 95 * a slowpath during the update.  After this function returns, all
 96 * subsequent calls to rcu_sync_is_idle() will return false, which
 97 * tells readers to stay off their fastpaths.  A later call to
 98 * rcu_sync_exit() re-enables reader fastpaths.
 99 *
100 * When called in isolation, rcu_sync_enter() must wait for a grace
101 * period, however, closely spaced calls to rcu_sync_enter() can
102 * optimize away the grace-period wait via a state machine implemented
103 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
104 */
105void rcu_sync_enter(struct rcu_sync *rsp)
106{
107	int gp_state;
108
109	spin_lock_irq(&rsp->rss_lock);
110	gp_state = rsp->gp_state;
111	if (gp_state == GP_IDLE) {
112		WRITE_ONCE(rsp->gp_state, GP_ENTER);
113		WARN_ON_ONCE(rsp->gp_count);
114		/*
115		 * Note that we could simply do rcu_sync_call(rsp) here and
116		 * avoid the "if (gp_state == GP_IDLE)" block below.
117		 *
118		 * However, synchronize_rcu() can be faster if rcu_expedited
119		 * or rcu_blocking_is_gp() is true.
120		 *
121		 * Another reason is that we can't wait for rcu callback if
122		 * we are called at early boot time but this shouldn't happen.
123		 */
124	}
125	rsp->gp_count++;
126	spin_unlock_irq(&rsp->rss_lock);
127
128	if (gp_state == GP_IDLE) {
129		/*
130		 * See the comment above, this simply does the "synchronous"
131		 * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED.
132		 */
133		synchronize_rcu();
134		rcu_sync_func(&rsp->cb_head);
135		/* Not really needed, wait_event() would see GP_PASSED. */
136		return;
137	}
138
139	wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED);
140}
141
142/**
143 * rcu_sync_exit() - Allow readers back onto fast path after grace period
144 * @rsp: Pointer to rcu_sync structure to use for synchronization
145 *
146 * This function is used by updaters who have completed, and can therefore
147 * now allow readers to make use of their fastpaths after a grace period
148 * has elapsed.  After this grace period has completed, all subsequent
149 * calls to rcu_sync_is_idle() will return true, which tells readers that
150 * they can once again use their fastpaths.
151 */
152void rcu_sync_exit(struct rcu_sync *rsp)
153{
154	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
155
156	spin_lock_irq(&rsp->rss_lock);
157	WARN_ON_ONCE(rsp->gp_count == 0);
158	if (!--rsp->gp_count) {
159		if (rsp->gp_state == GP_PASSED) {
160			WRITE_ONCE(rsp->gp_state, GP_EXIT);
161			rcu_sync_call(rsp);
162		} else if (rsp->gp_state == GP_EXIT) {
163			WRITE_ONCE(rsp->gp_state, GP_REPLAY);
164		}
165	}
166	spin_unlock_irq(&rsp->rss_lock);
167}
168
169/**
170 * rcu_sync_dtor() - Clean up an rcu_sync structure
171 * @rsp: Pointer to rcu_sync structure to be cleaned up
172 */
173void rcu_sync_dtor(struct rcu_sync *rsp)
174{
175	int gp_state;
176
177	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
178
179	spin_lock_irq(&rsp->rss_lock);
180	WARN_ON_ONCE(rsp->gp_count);
181	if (rsp->gp_state == GP_REPLAY)
182		WRITE_ONCE(rsp->gp_state, GP_EXIT);
183	gp_state = rsp->gp_state;
184	spin_unlock_irq(&rsp->rss_lock);
185
186	if (gp_state != GP_IDLE) {
187		rcu_barrier();
188		WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
189	}
190}