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	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
156
157	spin_lock_irq(&rsp->rss_lock);
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_count));
178	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
179
180	spin_lock_irq(&rsp->rss_lock);
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}
191