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