projects/linux·blob v4.3 · refs

178 lines · 5.3 KiB

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
11int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
12 const char *name, struct lock_class_key *rwsem_key)
13{
14 brw->fast_read_ctr = alloc_percpu(int);
15 if (unlikely(!brw->fast_read_ctr))
16 return -ENOMEM;
17
18 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
19 __init_rwsem(&brw->rw_sem, name, rwsem_key);
20 atomic_set(&brw->write_ctr, 0);
21 atomic_set(&brw->slow_read_ctr, 0);
22 init_waitqueue_head(&brw->write_waitq);
23 return 0;
24}
25
26void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
27{
28 free_percpu(brw->fast_read_ctr);
29 brw->fast_read_ctr = NULL; /* catch use after free bugs */
30}
31
32/*
33 * This is the fast-path for down_read/up_read, it only needs to ensure
34 * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
35 * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
36 * serialize with the preempt-disabled section below.
37 *
38 * The nontrivial part is that we should guarantee acquire/release semantics
39 * in case when
40 *
41 * R_W: down_write() comes after up_read(), the writer should see all
42 * changes done by the reader
43 * or
44 * W_R: down_read() comes after up_write(), the reader should see all
45 * changes done by the writer
46 *
47 * If this helper fails the callers rely on the normal rw_semaphore and
48 * atomic_dec_and_test(), so in this case we have the necessary barriers.
49 *
50 * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
51 * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
52 * reader inside the critical section. See the comments in down_write and
53 * up_write below.
54 */
55static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
56{
57 bool success = false;
58
59 preempt_disable();
60 if (likely(!atomic_read(&brw->write_ctr))) {
61 __this_cpu_add(*brw->fast_read_ctr, val);
62 success = true;
63 }
64 preempt_enable();
65
66 return success;
67}
68
69/*
70 * Like the normal down_read() this is not recursive, the writer can
71 * come after the first percpu_down_read() and create the deadlock.
72 *
73 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
74 * percpu_up_read() does rwsem_release(). This pairs with the usage
75 * of ->rw_sem in percpu_down/up_write().
76 */
77void percpu_down_read(struct percpu_rw_semaphore *brw)
78{
79 might_sleep();
80 if (likely(update_fast_ctr(brw, +1))) {
81 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
82 return;
83 }
84
85 down_read(&brw->rw_sem);
86 atomic_inc(&brw->slow_read_ctr);
87 /* avoid up_read()->rwsem_release() */
88 __up_read(&brw->rw_sem);
89}
90
91int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
92{
93 if (unlikely(!update_fast_ctr(brw, +1))) {
94 if (!__down_read_trylock(&brw->rw_sem))
95 return 0;
96 atomic_inc(&brw->slow_read_ctr);
97 __up_read(&brw->rw_sem);
98 }
99
100 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
101 return 1;
102}
103
104void percpu_up_read(struct percpu_rw_semaphore *brw)
105{
106 rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
107
108 if (likely(update_fast_ctr(brw, -1)))
109 return;
110
111 /* false-positive is possible but harmless */
112 if (atomic_dec_and_test(&brw->slow_read_ctr))
113 wake_up_all(&brw->write_waitq);
114}
115
116static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
117{
118 unsigned int sum = 0;
119 int cpu;
120
121 for_each_possible_cpu(cpu) {
122 sum += per_cpu(*brw->fast_read_ctr, cpu);
123 per_cpu(*brw->fast_read_ctr, cpu) = 0;
124 }
125
126 return sum;
127}
128
129/*
130 * A writer increments ->write_ctr to force the readers to switch to the
131 * slow mode, note the atomic_read() check in update_fast_ctr().
132 *
133 * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
134 * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
135 * counter it represents the number of active readers.
136 *
137 * Finally the writer takes ->rw_sem for writing and blocks the new readers,
138 * then waits until the slow counter becomes zero.
139 */
140void percpu_down_write(struct percpu_rw_semaphore *brw)
141{
142 /* tell update_fast_ctr() there is a pending writer */
143 atomic_inc(&brw->write_ctr);
144 /*
145 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
146 * so that update_fast_ctr() can't succeed.
147 *
148 * 2. Ensures we see the result of every previous this_cpu_add() in
149 * update_fast_ctr().
150 *
151 * 3. Ensures that if any reader has exited its critical section via
152 * fast-path, it executes a full memory barrier before we return.
153 * See R_W case in the comment above update_fast_ctr().
154 */
155 synchronize_sched_expedited();
156
157 /* exclude other writers, and block the new readers completely */
158 down_write(&brw->rw_sem);
159
160 /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
161 atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
162
163 /* wait for all readers to complete their percpu_up_read() */
164 wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
165}
166
167void percpu_up_write(struct percpu_rw_semaphore *brw)
168{
169 /* release the lock, but the readers can't use the fast-path */
170 up_write(&brw->rw_sem);
171 /*
172 * Insert the barrier before the next fast-path in down_read,
173 * see W_R case in the comment above update_fast_ctr().
174 */
175 synchronize_sched_expedited();
176 /* the last writer unblocks update_fast_ctr() */
177 atomic_dec(&brw->write_ctr);
178}