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1// SPDX-License-Identifier: GPL-2.0+
2/*
3 * Sleepable Read-Copy Update mechanism for mutual exclusion,
4 * tiny version for non-preemptible single-CPU use.
5 *
6 * Copyright (C) IBM Corporation, 2017
7 *
8 * Author: Paul McKenney <[email protected]>
9 */
10
11#include <linux/export.h>
12#include <linux/irq_work.h>
13#include <linux/mutex.h>
14#include <linux/preempt.h>
15#include <linux/rcupdate_wait.h>
16#include <linux/sched.h>
17#include <linux/delay.h>
18#include <linux/srcu.h>
19
20#include <linux/rcu_node_tree.h>
21#include "rcu_segcblist.h"
22#include "rcu.h"
23
24#ifndef CONFIG_TREE_RCU
25int rcu_scheduler_active __read_mostly;
26#else // #ifndef CONFIG_TREE_RCU
27extern int rcu_scheduler_active;
28#endif // #else // #ifndef CONFIG_TREE_RCU
29static LIST_HEAD(srcu_boot_list);
30static bool srcu_init_done;
31
32static int init_srcu_struct_fields(struct srcu_struct *ssp)
33{
34 ssp->srcu_lock_nesting[0] = 0;
35 ssp->srcu_lock_nesting[1] = 0;
36 init_swait_queue_head(&ssp->srcu_wq);
37 ssp->srcu_cb_head = NULL;
38 ssp->srcu_cb_tail = &ssp->srcu_cb_head;
39 ssp->srcu_gp_running = false;
40 ssp->srcu_gp_waiting = false;
41 ssp->srcu_idx = 0;
42 ssp->srcu_idx_max = 0;
43 INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
44 INIT_LIST_HEAD(&ssp->srcu_work.entry);
45 init_irq_work(&ssp->srcu_irq_work, srcu_tiny_irq_work);
46 return 0;
47}
48
49#ifdef CONFIG_DEBUG_LOCK_ALLOC
50
51int init_srcu_struct_lockdep(struct srcu_struct *ssp, const char *name,
52 struct lock_class_key *key)
53{
54 /* Don't re-initialize a lock while it is held. */
55 debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
56 lockdep_init_map(&ssp->dep_map, name, key, 0);
57 return init_srcu_struct_fields(ssp);
58}
59EXPORT_SYMBOL_GPL(init_srcu_struct_lockdep);
60
61#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
62
63/*
64 * init_srcu_struct_generic - initialize a sleep-RCU structure
65 * @ssp: structure to initialize.
66 *
67 * Must invoke this on a given srcu_struct before passing that srcu_struct
68 * to any other function. Each srcu_struct represents a separate domain
69 * of SRCU protection.
70 */
71int init_srcu_struct_generic(struct srcu_struct *ssp)
72{
73 return init_srcu_struct_fields(ssp);
74}
75EXPORT_SYMBOL_GPL(init_srcu_struct_generic);
76
77#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
78
79/*
80 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
81 * @ssp: structure to clean up.
82 *
83 * Must invoke this after you are finished using a given srcu_struct that
84 * was initialized via init_srcu_struct(), else you leak memory.
85 */
86void cleanup_srcu_struct(struct srcu_struct *ssp)
87{
88 WARN_ON(srcu_readers_active(ssp));
89 irq_work_sync(&ssp->srcu_irq_work);
90 flush_work(&ssp->srcu_work);
91 WARN_ON(ssp->srcu_gp_running);
92 WARN_ON(ssp->srcu_gp_waiting);
93 WARN_ON(ssp->srcu_cb_head);
94 WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
95 WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
96 WARN_ON(ssp->srcu_idx & 0x1);
97}
98EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
99
100/*
101 * Removes the count for the old reader from the appropriate element of
102 * the srcu_struct.
103 */
104void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
105{
106 int newval;
107
108 preempt_disable(); // Needed for PREEMPT_LAZY
109 newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1;
110 WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
111 preempt_enable();
112 if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task() && !irqs_disabled())
113 swake_up_one(&ssp->srcu_wq);
114}
115EXPORT_SYMBOL_GPL(__srcu_read_unlock);
116
117/*
118 * Workqueue handler to drive one grace period and invoke any callbacks
119 * that become ready as a result. Single-CPU operation and preemption
120 * disabling mean that we get away with murder on synchronization. ;-)
121 */
122void srcu_drive_gp(struct work_struct *wp)
123{
124 int idx;
125 struct rcu_head *lh;
126 struct rcu_head *rhp;
127 struct srcu_struct *ssp;
128
129 ssp = container_of(wp, struct srcu_struct, srcu_work);
130 preempt_disable(); // Needed for PREEMPT_LAZY
131 if (ssp->srcu_gp_running || ULONG_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) {
132 preempt_enable();
133 return; /* Already running or nothing to do. */
134 }
135
136 /* Remove recently arrived callbacks and wait for readers. */
137 WRITE_ONCE(ssp->srcu_gp_running, true);
138 local_irq_disable();
139 lh = ssp->srcu_cb_head;
140 ssp->srcu_cb_head = NULL;
141 ssp->srcu_cb_tail = &ssp->srcu_cb_head;
142 local_irq_enable();
143 idx = (ssp->srcu_idx & 0x2) / 2;
144 WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
145 WRITE_ONCE(ssp->srcu_gp_waiting, true); /* srcu_read_unlock() wakes! */
146 preempt_enable();
147 do {
148 // Deadlock issues prevent __srcu_read_unlock() from
149 // doing an unconditional wakeup, so polling is required.
150 swait_event_timeout_exclusive(ssp->srcu_wq,
151 !READ_ONCE(ssp->srcu_lock_nesting[idx]), HZ / 10);
152 } while (READ_ONCE(ssp->srcu_lock_nesting[idx]));
153 preempt_disable(); // Needed for PREEMPT_LAZY
154 WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
155 WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
156 preempt_enable();
157
158 /* Invoke the callbacks we removed above. */
159 while (lh) {
160 rhp = lh;
161 lh = lh->next;
162 debug_rcu_head_callback(rhp);
163 local_bh_disable();
164 rhp->func(rhp);
165 local_bh_enable();
166 }
167
168 /*
169 * Enable rescheduling, and if there are more callbacks,
170 * reschedule ourselves. This can race with a call_srcu()
171 * at interrupt level, but the ->srcu_gp_running checks will
172 * straighten that out.
173 */
174 preempt_disable(); // Needed for PREEMPT_LAZY
175 WRITE_ONCE(ssp->srcu_gp_running, false);
176 idx = ULONG_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max));
177 preempt_enable();
178 if (idx)
179 schedule_work(&ssp->srcu_work);
180}
181EXPORT_SYMBOL_GPL(srcu_drive_gp);
182
183/*
184 * Use an irq_work to defer schedule_work() to avoid acquiring the workqueue
185 * pool->lock while the caller might hold scheduler locks, causing lockdep
186 * splats due to workqueue_init() doing a wakeup.
187 */
188void srcu_tiny_irq_work(struct irq_work *irq_work)
189{
190 struct srcu_struct *ssp;
191
192 ssp = container_of(irq_work, struct srcu_struct, srcu_irq_work);
193 schedule_work(&ssp->srcu_work);
194}
195EXPORT_SYMBOL_GPL(srcu_tiny_irq_work);
196
197static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
198{
199 unsigned long cookie;
200
201 lockdep_assert_preemption_disabled(); // Needed for PREEMPT_LAZY
202 cookie = get_state_synchronize_srcu(ssp);
203 if (ULONG_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) {
204 return;
205 }
206 WRITE_ONCE(ssp->srcu_idx_max, cookie);
207 if (!READ_ONCE(ssp->srcu_gp_running)) {
208 if (likely(srcu_init_done))
209 irq_work_queue(&ssp->srcu_irq_work);
210 else if (list_empty(&ssp->srcu_work.entry))
211 list_add(&ssp->srcu_work.entry, &srcu_boot_list);
212 }
213}
214
215/*
216 * Enqueue an SRCU callback on the specified srcu_struct structure,
217 * initiating grace-period processing if it is not already running.
218 */
219void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
220 rcu_callback_t func)
221{
222 unsigned long flags;
223
224 rhp->func = func;
225 rhp->next = NULL;
226 preempt_disable(); // Needed for PREEMPT_LAZY
227 local_irq_save(flags);
228 *ssp->srcu_cb_tail = rhp;
229 ssp->srcu_cb_tail = &rhp->next;
230 local_irq_restore(flags);
231 srcu_gp_start_if_needed(ssp);
232 preempt_enable();
233}
234EXPORT_SYMBOL_GPL(call_srcu);
235
236/*
237 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
238 */
239void synchronize_srcu(struct srcu_struct *ssp)
240{
241 struct rcu_synchronize rs;
242
243 srcu_lock_sync(&ssp->dep_map);
244
245 RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
246 lock_is_held(&rcu_bh_lock_map) ||
247 lock_is_held(&rcu_lock_map) ||
248 lock_is_held(&rcu_sched_lock_map),
249 "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
250
251 if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
252 return;
253
254 might_sleep();
255 init_rcu_head_on_stack(&rs.head);
256 init_completion(&rs.completion);
257 call_srcu(ssp, &rs.head, wakeme_after_rcu);
258 wait_for_completion(&rs.completion);
259 destroy_rcu_head_on_stack(&rs.head);
260}
261EXPORT_SYMBOL_GPL(synchronize_srcu);
262
263/*
264 * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
265 */
266unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
267{
268 unsigned long ret;
269
270 barrier();
271 ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
272 barrier();
273 return ret;
274}
275EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
276
277/*
278 * start_poll_synchronize_srcu - Provide cookie and start grace period
279 *
280 * The difference between this and get_state_synchronize_srcu() is that
281 * this function ensures that the poll_state_synchronize_srcu() will
282 * eventually return the value true.
283 */
284unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
285{
286 unsigned long ret;
287
288 preempt_disable(); // Needed for PREEMPT_LAZY
289 ret = get_state_synchronize_srcu(ssp);
290 srcu_gp_start_if_needed(ssp);
291 preempt_enable();
292 return ret;
293}
294EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
295
296/*
297 * poll_state_synchronize_srcu - Has cookie's grace period ended?
298 */
299bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
300{
301 unsigned long cur_s = READ_ONCE(ssp->srcu_idx);
302
303 barrier();
304 return cookie == SRCU_GET_STATE_COMPLETED ||
305 ULONG_CMP_GE(cur_s, cookie) || ULONG_CMP_LT(cur_s, cookie - 3);
306}
307EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
308
309#ifndef CONFIG_TREE_RCU
310/* Lockdep diagnostics. */
311void __init rcu_scheduler_starting(void)
312{
313 rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
314}
315#endif // #ifndef CONFIG_TREE_RCU
316
317/*
318 * Queue work for srcu_struct structures with early boot callbacks.
319 * The work won't actually execute until the workqueue initialization
320 * phase that takes place after the scheduler starts.
321 */
322void __init srcu_init(void)
323{
324 struct srcu_struct *ssp;
325
326 srcu_init_done = true;
327 while (!list_empty(&srcu_boot_list)) {
328 ssp = list_first_entry(&srcu_boot_list,
329 struct srcu_struct, srcu_work.entry);
330 list_del_init(&ssp->srcu_work.entry);
331 schedule_work(&ssp->srcu_work);
332 }
333}