Merge tag 'xfs-5.10-merge-7' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux
[linux-2.6-microblaze.git] / kernel / locking / percpu-rwsem.c
1 // SPDX-License-Identifier: GPL-2.0-only
2 #include <linux/atomic.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/sched/task.h>
10 #include <linux/errno.h>
11
12 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
13                         const char *name, struct lock_class_key *key)
14 {
15         sem->read_count = alloc_percpu(int);
16         if (unlikely(!sem->read_count))
17                 return -ENOMEM;
18
19         rcu_sync_init(&sem->rss);
20         rcuwait_init(&sem->writer);
21         init_waitqueue_head(&sem->waiters);
22         atomic_set(&sem->block, 0);
23 #ifdef CONFIG_DEBUG_LOCK_ALLOC
24         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
25         lockdep_init_map(&sem->dep_map, name, key, 0);
26 #endif
27         return 0;
28 }
29 EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
30
31 void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
32 {
33         /*
34          * XXX: temporary kludge. The error path in alloc_super()
35          * assumes that percpu_free_rwsem() is safe after kzalloc().
36          */
37         if (!sem->read_count)
38                 return;
39
40         rcu_sync_dtor(&sem->rss);
41         free_percpu(sem->read_count);
42         sem->read_count = NULL; /* catch use after free bugs */
43 }
44 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
45
46 static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
47 {
48         this_cpu_inc(*sem->read_count);
49
50         /*
51          * Due to having preemption disabled the decrement happens on
52          * the same CPU as the increment, avoiding the
53          * increment-on-one-CPU-and-decrement-on-another problem.
54          *
55          * If the reader misses the writer's assignment of sem->block, then the
56          * writer is guaranteed to see the reader's increment.
57          *
58          * Conversely, any readers that increment their sem->read_count after
59          * the writer looks are guaranteed to see the sem->block value, which
60          * in turn means that they are guaranteed to immediately decrement
61          * their sem->read_count, so that it doesn't matter that the writer
62          * missed them.
63          */
64
65         smp_mb(); /* A matches D */
66
67         /*
68          * If !sem->block the critical section starts here, matched by the
69          * release in percpu_up_write().
70          */
71         if (likely(!atomic_read_acquire(&sem->block)))
72                 return true;
73
74         this_cpu_dec(*sem->read_count);
75
76         /* Prod writer to re-evaluate readers_active_check() */
77         rcuwait_wake_up(&sem->writer);
78
79         return false;
80 }
81
82 static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
83 {
84         if (atomic_read(&sem->block))
85                 return false;
86
87         return atomic_xchg(&sem->block, 1) == 0;
88 }
89
90 static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
91 {
92         if (reader) {
93                 bool ret;
94
95                 preempt_disable();
96                 ret = __percpu_down_read_trylock(sem);
97                 preempt_enable();
98
99                 return ret;
100         }
101         return __percpu_down_write_trylock(sem);
102 }
103
104 /*
105  * The return value of wait_queue_entry::func means:
106  *
107  *  <0 - error, wakeup is terminated and the error is returned
108  *   0 - no wakeup, a next waiter is tried
109  *  >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
110  *
111  * We use EXCLUSIVE for both readers and writers to preserve FIFO order,
112  * and play games with the return value to allow waking multiple readers.
113  *
114  * Specifically, we wake readers until we've woken a single writer, or until a
115  * trylock fails.
116  */
117 static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
118                                       unsigned int mode, int wake_flags,
119                                       void *key)
120 {
121         bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
122         struct percpu_rw_semaphore *sem = key;
123         struct task_struct *p;
124
125         /* concurrent against percpu_down_write(), can get stolen */
126         if (!__percpu_rwsem_trylock(sem, reader))
127                 return 1;
128
129         p = get_task_struct(wq_entry->private);
130         list_del_init(&wq_entry->entry);
131         smp_store_release(&wq_entry->private, NULL);
132
133         wake_up_process(p);
134         put_task_struct(p);
135
136         return !reader; /* wake (readers until) 1 writer */
137 }
138
139 static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
140 {
141         DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
142         bool wait;
143
144         spin_lock_irq(&sem->waiters.lock);
145         /*
146          * Serialize against the wakeup in percpu_up_write(), if we fail
147          * the trylock, the wakeup must see us on the list.
148          */
149         wait = !__percpu_rwsem_trylock(sem, reader);
150         if (wait) {
151                 wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
152                 __add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
153         }
154         spin_unlock_irq(&sem->waiters.lock);
155
156         while (wait) {
157                 set_current_state(TASK_UNINTERRUPTIBLE);
158                 if (!smp_load_acquire(&wq_entry.private))
159                         break;
160                 schedule();
161         }
162         __set_current_state(TASK_RUNNING);
163 }
164
165 bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
166 {
167         if (__percpu_down_read_trylock(sem))
168                 return true;
169
170         if (try)
171                 return false;
172
173         preempt_enable();
174         percpu_rwsem_wait(sem, /* .reader = */ true);
175         preempt_disable();
176
177         return true;
178 }
179 EXPORT_SYMBOL_GPL(__percpu_down_read);
180
181 #define per_cpu_sum(var)                                                \
182 ({                                                                      \
183         typeof(var) __sum = 0;                                          \
184         int cpu;                                                        \
185         compiletime_assert_atomic_type(__sum);                          \
186         for_each_possible_cpu(cpu)                                      \
187                 __sum += per_cpu(var, cpu);                             \
188         __sum;                                                          \
189 })
190
191 /*
192  * Return true if the modular sum of the sem->read_count per-CPU variable is
193  * zero.  If this sum is zero, then it is stable due to the fact that if any
194  * newly arriving readers increment a given counter, they will immediately
195  * decrement that same counter.
196  *
197  * Assumes sem->block is set.
198  */
199 static bool readers_active_check(struct percpu_rw_semaphore *sem)
200 {
201         if (per_cpu_sum(*sem->read_count) != 0)
202                 return false;
203
204         /*
205          * If we observed the decrement; ensure we see the entire critical
206          * section.
207          */
208
209         smp_mb(); /* C matches B */
210
211         return true;
212 }
213
214 void percpu_down_write(struct percpu_rw_semaphore *sem)
215 {
216         might_sleep();
217         rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
218
219         /* Notify readers to take the slow path. */
220         rcu_sync_enter(&sem->rss);
221
222         /*
223          * Try set sem->block; this provides writer-writer exclusion.
224          * Having sem->block set makes new readers block.
225          */
226         if (!__percpu_down_write_trylock(sem))
227                 percpu_rwsem_wait(sem, /* .reader = */ false);
228
229         /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
230
231         /*
232          * If they don't see our store of sem->block, then we are guaranteed to
233          * see their sem->read_count increment, and therefore will wait for
234          * them.
235          */
236
237         /* Wait for all active readers to complete. */
238         rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
239 }
240 EXPORT_SYMBOL_GPL(percpu_down_write);
241
242 void percpu_up_write(struct percpu_rw_semaphore *sem)
243 {
244         rwsem_release(&sem->dep_map, _RET_IP_);
245
246         /*
247          * Signal the writer is done, no fast path yet.
248          *
249          * One reason that we cannot just immediately flip to readers_fast is
250          * that new readers might fail to see the results of this writer's
251          * critical section.
252          *
253          * Therefore we force it through the slow path which guarantees an
254          * acquire and thereby guarantees the critical section's consistency.
255          */
256         atomic_set_release(&sem->block, 0);
257
258         /*
259          * Prod any pending reader/writer to make progress.
260          */
261         __wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
262
263         /*
264          * Once this completes (at least one RCU-sched grace period hence) the
265          * reader fast path will be available again. Safe to use outside the
266          * exclusive write lock because its counting.
267          */
268         rcu_sync_exit(&sem->rss);
269 }
270 EXPORT_SYMBOL_GPL(percpu_up_write);