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