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2023-12-08 01573e231f18eb2d99162747186f59511f56b64d 
 kernel/kernel/rcu/sync.c
....@@ -1,19 +1,6 @@
1
+// SPDX-License-Identifier: GPL-2.0+
12 /*
23  * RCU-based infrastructure for lightweight reader-writer locking
3
- *
4
- * This program is free software; you can redistribute it and/or modify
5
- * it under the terms of the GNU General Public License as published by
6
- * the Free Software Foundation; either version 2 of the License, or
7
- * (at your option) any later version.
8
- *
9
- * This program is distributed in the hope that it will be useful,
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- * but WITHOUT ANY WARRANTY; without even the implied warranty of
11
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12
- * GNU General Public License for more details.
13
- *
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- * You should have received a copy of the GNU General Public License
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- * along with this program; if not, you can access it online at
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- * http://www.gnu.org/licenses/gpl-2.0.html.
174  *
185  * Copyright (c) 2015, Red Hat, Inc.
196  *
....@@ -23,65 +10,18 @@
2310 #include <linux/rcu_sync.h>
2411 #include <linux/sched.h>
2512 
26
-#ifdef CONFIG_PROVE_RCU
27
-#define __INIT_HELD(func)	.held = func,
28
-#else
29
-#define __INIT_HELD(func)
30
-#endif
31
-
32
-static const struct {
33
-	void (*sync)(void);
34
-	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
35
-	void (*wait)(void);
36
-#ifdef CONFIG_PROVE_RCU
37
-	int  (*held)(void);
38
-#endif
39
-} gp_ops[] = {
40
-	[RCU_SYNC] = {
41
-		.sync = synchronize_rcu,
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-		.call = call_rcu,
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-		.wait = rcu_barrier,
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-		__INIT_HELD(rcu_read_lock_held)
45
-	},
46
-	[RCU_SCHED_SYNC] = {
47
-		.sync = synchronize_sched,
48
-		.call = call_rcu_sched,
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-		.wait = rcu_barrier_sched,
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-		__INIT_HELD(rcu_read_lock_sched_held)
51
-	},
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-	[RCU_BH_SYNC] = {
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-		.sync = synchronize_rcu_bh,
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-		.call = call_rcu_bh,
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-		.wait = rcu_barrier_bh,
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-		__INIT_HELD(rcu_read_lock_bh_held)
57
-	},
58
-};
59
-
60
-enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
61
-enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
13
+enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY };
6214 
6315 #define	rss_lock	gp_wait.lock
64
-
65
-#ifdef CONFIG_PROVE_RCU
66
-void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
67
-{
68
-	RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
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-			 "suspicious rcu_sync_is_idle() usage");
70
-}
71
-
72
-EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
73
-#endif
7416 
7517 /**
7618  * rcu_sync_init() - Initialize an rcu_sync structure
7719  * @rsp: Pointer to rcu_sync structure to be initialized
78
- * @type: Flavor of RCU with which to synchronize rcu_sync structure
7920  */
80
-void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
21
+void rcu_sync_init(struct rcu_sync *rsp)
8122 {
8223 	memset(rsp, 0, sizeof(*rsp));
8324 	init_waitqueue_head(&rsp->gp_wait);
84
-	rsp->gp_type = type;
8525 }
8626 
8727 /**
....@@ -97,6 +37,70 @@
9737 {
9838 	rsp->gp_count++;
9939 	rsp->gp_state = GP_PASSED;
40
+}
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+
42
+
43
+static void rcu_sync_func(struct rcu_head *rhp);
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+
45
+static void rcu_sync_call(struct rcu_sync *rsp)
46
+{
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+	call_rcu(&rsp->cb_head, rcu_sync_func);
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+}
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+
50
+/**
51
+ * 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.
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()
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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
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
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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.
72
+ */
73
+static 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);
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+	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);
100104 }
101105 
102106 /**
....@@ -116,85 +120,43 @@
116120  */
117121 void rcu_sync_enter(struct rcu_sync *rsp)
118122 {
119
-	bool need_wait, need_sync;
123
+	int gp_state;
120124 
121125 	spin_lock_irq(&rsp->rss_lock);
122
-	need_wait = rsp->gp_count++;
123
-	need_sync = rsp->gp_state == GP_IDLE;
124
-	if (need_sync)
125
-		rsp->gp_state = GP_PENDING;
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
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+		 * avoid the "if (gp_state == GP_IDLE)" block below.
133
+		 *
134
+		 * However, synchronize_rcu() can be faster if rcu_expedited
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+		 * or rcu_blocking_is_gp() is true.
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+		 *
137
+		 * 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.
139
+		 */
140
+	}
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+	rsp->gp_count++;
126142 	spin_unlock_irq(&rsp->rss_lock);
127143 
128
-	BUG_ON(need_wait && need_sync);
129
-
130
-	if (need_sync) {
131
-		gp_ops[rsp->gp_type].sync();
132
-		rsp->gp_state = GP_PASSED;
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-		wake_up_all(&rsp->gp_wait);
134
-	} else if (need_wait) {
135
-		wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
136
-	} else {
144
+	if (gp_state == GP_IDLE) {
137145 		/*
138
-		 * Possible when there's a pending CB from a rcu_sync_exit().
139
-		 * Nobody has yet been allowed the 'fast' path and thus we can
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-		 * avoid doing any sync(). The callback will get 'dropped'.
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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.
141148 		 */
142
-		BUG_ON(rsp->gp_state != GP_PASSED);
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+		synchronize_rcu();
150
+		rcu_sync_func(&rsp->cb_head);
151
+		/* Not really needed, wait_event() would see GP_PASSED. */
152
+		return;
143153 	}
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+
155
+	wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED);
144156 }
145157 
146158 /**
147
- * rcu_sync_func() - Callback function managing reader access to fastpath
148
- * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
149
- *
150
- * This function is passed to one of the call_rcu() functions by
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- * rcu_sync_exit(), so that it is invoked after a grace period following the
152
- * that invocation of rcu_sync_exit().  It takes action based on events that
153
- * have taken place in the meantime, so that closely spaced rcu_sync_enter()
154
- * and rcu_sync_exit() pairs need not wait for a grace period.
155
- *
156
- * If another rcu_sync_enter() is invoked before the grace period
157
- * ended, reset state to allow the next rcu_sync_exit() to let the
158
- * readers back onto their fastpaths (after a grace period).  If both
159
- * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
160
- * before the grace period ended, re-invoke call_rcu() on behalf of that
161
- * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
162
- * can again use their fastpaths.
163
- */
164
-static void rcu_sync_func(struct rcu_head *rhp)
165
-{
166
-	struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
167
-	unsigned long flags;
168
-
169
-	BUG_ON(rsp->gp_state != GP_PASSED);
170
-	BUG_ON(rsp->cb_state == CB_IDLE);
171
-
172
-	spin_lock_irqsave(&rsp->rss_lock, flags);
173
-	if (rsp->gp_count) {
174
-		/*
175
-		 * A new rcu_sync_begin() has happened; drop the callback.
176
-		 */
177
-		rsp->cb_state = CB_IDLE;
178
-	} else if (rsp->cb_state == CB_REPLAY) {
179
-		/*
180
-		 * A new rcu_sync_exit() has happened; requeue the callback
181
-		 * to catch a later GP.
182
-		 */
183
-		rsp->cb_state = CB_PENDING;
184
-		gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
185
-	} else {
186
-		/*
187
-		 * We're at least a GP after rcu_sync_exit(); eveybody will now
188
-		 * have observed the write side critical section. Let 'em rip!.
189
-		 */
190
-		rsp->cb_state = CB_IDLE;
191
-		rsp->gp_state = GP_IDLE;
192
-	}
193
-	spin_unlock_irqrestore(&rsp->rss_lock, flags);
194
-}
195
-
196
-/**
197
- * rcu_sync_exit() - Allow readers back onto fast patch after grace period
159
+ * rcu_sync_exit() - Allow readers back onto fast path after grace period
198160  * @rsp: Pointer to rcu_sync structure to use for synchronization
199161  *
200162  * This function is used by updaters who have completed, and can therefore
....@@ -205,13 +167,16 @@
205167  */
206168 void rcu_sync_exit(struct rcu_sync *rsp)
207169 {
170
+	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
171
+	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
172
+
208173 	spin_lock_irq(&rsp->rss_lock);
209174 	if (!--rsp->gp_count) {
210
-		if (rsp->cb_state == CB_IDLE) {
211
-			rsp->cb_state = CB_PENDING;
212
-			gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
213
-		} else if (rsp->cb_state == CB_PENDING) {
214
-			rsp->cb_state = CB_REPLAY;
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);
215180 		}
216181 	}
217182 	spin_unlock_irq(&rsp->rss_lock);
....@@ -223,18 +188,19 @@
223188  */
224189 void rcu_sync_dtor(struct rcu_sync *rsp)
225190 {
226
-	int cb_state;
191
+	int gp_state;
227192 
228
-	BUG_ON(rsp->gp_count);
193
+	WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
194
+	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
229195 
230196 	spin_lock_irq(&rsp->rss_lock);
231
-	if (rsp->cb_state == CB_REPLAY)
232
-		rsp->cb_state = CB_PENDING;
233
-	cb_state = rsp->cb_state;
197
+	if (rsp->gp_state == GP_REPLAY)
198
+		WRITE_ONCE(rsp->gp_state, GP_EXIT);
199
+	gp_state = rsp->gp_state;
234200 	spin_unlock_irq(&rsp->rss_lock);
235201 
236
-	if (cb_state != CB_IDLE) {
237
-		gp_ops[rsp->gp_type].wait();
238
-		BUG_ON(rsp->cb_state != CB_IDLE);
202
+	if (gp_state != GP_IDLE) {
203
+		rcu_barrier();
204
+		WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
239205 	}
240206 }