1 // SPDX-License-Identifier: GPL-2.0-only
2 #include <linux/atomic.h>
3 #include <linux/rwsem.h>
4 #include <linux/percpu.h>
5 #include <linux/lockdep.h>
6 #include <linux/percpu-rwsem.h>
7 #include <linux/rcupdate.h>
8 #include <linux/sched.h>
9 #include <linux/errno.h>
10
11 #include "rwsem.h"
12
13 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
14 const char *name, struct lock_class_key *rwsem_key)
15 {
16 sem->read_count = alloc_percpu(int);
17 if (unlikely(!sem->read_count))
18 return -ENOMEM;
19
20 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
21 rcu_sync_init(&sem->rss);
22 __init_rwsem(&sem->rw_sem, name, rwsem_key);
23 rcuwait_init(&sem->writer);
24 sem->readers_block = 0;
25 return 0;
26 }
27 EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
28
29 void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
30 {
31 /*
32 * XXX: temporary kludge. The error path in alloc_super()
33 * assumes that percpu_free_rwsem() is safe after kzalloc().
34 */
35 if (!sem->read_count)
36 return;
37
38 rcu_sync_dtor(&sem->rss);
39 free_percpu(sem->read_count);
40 sem->read_count = NULL; /* catch use after free bugs */
41 }
42 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
43
44 int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
45 {
46 /*
47 * Due to having preemption disabled the decrement happens on
48 * the same CPU as the increment, avoiding the
49 * increment-on-one-CPU-and-decrement-on-another problem.
50 *
51 * If the reader misses the writer's assignment of readers_block, then
52 * the writer is guaranteed to see the reader's increment.
53 *
54 * Conversely, any readers that increment their sem->read_count after
55 * the writer looks are guaranteed to see the readers_block value,
56 * which in turn means that they are guaranteed to immediately
57 * decrement their sem->read_count, so that it doesn't matter that the
58 * writer missed them.
59 */
60
61 smp_mb(); /* A matches D */
62
63 /*
64 * If !readers_block the critical section starts here, matched by the
65 * release in percpu_up_write().
66 */
67 if (likely(!smp_load_acquire(&sem->readers_block)))
68 return 1;
69
70 /*
71 * Per the above comment; we still have preemption disabled and
72 * will thus decrement on the same CPU as we incremented.
73 */
74 __percpu_up_read(sem);
75
76 if (try)
77 return 0;
78
79 /*
80 * We either call schedule() in the wait, or we'll fall through
81 * and reschedule on the preempt_enable() in percpu_down_read().
82 */
83 preempt_enable_no_resched();
84
85 /*
86 * Avoid lockdep for the down/up_read() we already have them.
87 */
88 __down_read(&sem->rw_sem);
89 this_cpu_inc(*sem->read_count);
90 __up_read(&sem->rw_sem);
91
92 preempt_disable();
93 return 1;
94 }
95 EXPORT_SYMBOL_GPL(__percpu_down_read);
96
97 void __percpu_up_read(struct percpu_rw_semaphore *sem)
98 {
99 smp_mb(); /* B matches C */
100 /*
101 * In other words, if they see our decrement (presumably to aggregate
102 * zero, as that is the only time it matters) they will also see our
103 * critical section.
104 */
105 __this_cpu_dec(*sem->read_count);
106
107 /* Prod writer to recheck readers_active */
108 rcuwait_wake_up(&sem->writer);
109 }
110 EXPORT_SYMBOL_GPL(__percpu_up_read);
111
112 #define per_cpu_sum(var) \
113 ({ \
114 typeof(var) __sum = 0; \
115 int cpu; \
116 compiletime_assert_atomic_type(__sum); \
117 for_each_possible_cpu(cpu) \
118 __sum += per_cpu(var, cpu); \
119 __sum; \
120 })
121
122 /*
123 * Return true if the modular sum of the sem->read_count per-CPU variable is
124 * zero. If this sum is zero, then it is stable due to the fact that if any
125 * newly arriving readers increment a given counter, they will immediately
126 * decrement that same counter.
127 */
128 static bool readers_active_check(struct percpu_rw_semaphore *sem)
129 {
130 if (per_cpu_sum(*sem->read_count) != 0)
131 return false;
132
133 /*
134 * If we observed the decrement; ensure we see the entire critical
135 * section.
136 */
137
138 smp_mb(); /* C matches B */
139
140 return true;
141 }
142
143 void percpu_down_write(struct percpu_rw_semaphore *sem)
144 {
145 /* Notify readers to take the slow path. */
146 rcu_sync_enter(&sem->rss);
147
148 down_write(&sem->rw_sem);
149
150 /*
151 * Notify new readers to block; up until now, and thus throughout the
152 * longish rcu_sync_enter() above, new readers could still come in.
153 */
154 WRITE_ONCE(sem->readers_block, 1);
155
156 smp_mb(); /* D matches A */
157
158 /*
159 * If they don't see our writer of readers_block, then we are
160 * guaranteed to see their sem->read_count increment, and therefore
161 * will wait for them.
162 */
163
164 /* Wait for all now active readers to complete. */
165 rcuwait_wait_event(&sem->writer, readers_active_check(sem));
166 }
167 EXPORT_SYMBOL_GPL(percpu_down_write);
168
169 void percpu_up_write(struct percpu_rw_semaphore *sem)
170 {
171 /*
172 * Signal the writer is done, no fast path yet.
173 *
174 * One reason that we cannot just immediately flip to readers_fast is
175 * that new readers might fail to see the results of this writer's
176 * critical section.
177 *
178 * Therefore we force it through the slow path which guarantees an
179 * acquire and thereby guarantees the critical section's consistency.
180 */
181 smp_store_release(&sem->readers_block, 0);
182
183 /*
184 * Release the write lock, this will allow readers back in the game.
185 */
186 up_write(&sem->rw_sem);
187
188 /*
189 * Once this completes (at least one RCU-sched grace period hence) the
190 * reader fast path will be available again. Safe to use outside the
191 * exclusive write lock because its counting.
192 */
193 rcu_sync_exit(&sem->rss);
194 }
195 EXPORT_SYMBOL_GPL(percpu_up_write);