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