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
13 enum { 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 */
rcu_sync_init(struct rcu_sync * rsp)21 void rcu_sync_init(struct rcu_sync *rsp)
22 {
23 memset(rsp, 0, sizeof(*rsp));
24 init_waitqueue_head(&rsp->gp_wait);
25 }
26
27 static void rcu_sync_func(struct rcu_head *rhp);
28
rcu_sync_call(struct rcu_sync * rsp)29 static 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 */
rcu_sync_func(struct rcu_head * rhp)57 static 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 */
rcu_sync_enter(struct rcu_sync * rsp)105 void 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 */
rcu_sync_exit(struct rcu_sync * rsp)152 void 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 */
rcu_sync_dtor(struct rcu_sync * rsp)173 void 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 }
191