Linux 6.7-rc7
[linux-modified.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/sched/debug.h>
11 #include <linux/errno.h>
12 #include <trace/events/lock.h>
13
14 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
15                         const char *name, struct lock_class_key *key)
16 {
17         sem->read_count = alloc_percpu(int);
18         if (unlikely(!sem->read_count))
19                 return -ENOMEM;
20
21         rcu_sync_init(&sem->rss);
22         rcuwait_init(&sem->writer);
23         init_waitqueue_head(&sem->waiters);
24         atomic_set(&sem->block, 0);
25 #ifdef CONFIG_DEBUG_LOCK_ALLOC
26         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
27         lockdep_init_map(&sem->dep_map, name, key, 0);
28 #endif
29         return 0;
30 }
31 EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
32
33 void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
34 {
35         /*
36          * XXX: temporary kludge. The error path in alloc_super()
37          * assumes that percpu_free_rwsem() is safe after kzalloc().
38          */
39         if (!sem->read_count)
40                 return;
41
42         rcu_sync_dtor(&sem->rss);
43         free_percpu(sem->read_count);
44         sem->read_count = NULL; /* catch use after free bugs */
45 }
46 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
47
48 static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
49 {
50         this_cpu_inc(*sem->read_count);
51
52         /*
53          * Due to having preemption disabled the decrement happens on
54          * the same CPU as the increment, avoiding the
55          * increment-on-one-CPU-and-decrement-on-another problem.
56          *
57          * If the reader misses the writer's assignment of sem->block, then the
58          * writer is guaranteed to see the reader's increment.
59          *
60          * Conversely, any readers that increment their sem->read_count after
61          * the writer looks are guaranteed to see the sem->block value, which
62          * in turn means that they are guaranteed to immediately decrement
63          * their sem->read_count, so that it doesn't matter that the writer
64          * missed them.
65          */
66
67         smp_mb(); /* A matches D */
68
69         /*
70          * If !sem->block the critical section starts here, matched by the
71          * release in percpu_up_write().
72          */
73         if (likely(!atomic_read_acquire(&sem->block)))
74                 return true;
75
76         this_cpu_dec(*sem->read_count);
77
78         /* Prod writer to re-evaluate readers_active_check() */
79         rcuwait_wake_up(&sem->writer);
80
81         return false;
82 }
83
84 static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
85 {
86         if (atomic_read(&sem->block))
87                 return false;
88
89         return atomic_xchg(&sem->block, 1) == 0;
90 }
91
92 static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
93 {
94         if (reader) {
95                 bool ret;
96
97                 preempt_disable();
98                 ret = __percpu_down_read_trylock(sem);
99                 preempt_enable();
100
101                 return ret;
102         }
103         return __percpu_down_write_trylock(sem);
104 }
105
106 /*
107  * The return value of wait_queue_entry::func means:
108  *
109  *  <0 - error, wakeup is terminated and the error is returned
110  *   0 - no wakeup, a next waiter is tried
111  *  >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
112  *
113  * We use EXCLUSIVE for both readers and writers to preserve FIFO order,
114  * and play games with the return value to allow waking multiple readers.
115  *
116  * Specifically, we wake readers until we've woken a single writer, or until a
117  * trylock fails.
118  */
119 static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
120                                       unsigned int mode, int wake_flags,
121                                       void *key)
122 {
123         bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
124         struct percpu_rw_semaphore *sem = key;
125         struct task_struct *p;
126
127         /* concurrent against percpu_down_write(), can get stolen */
128         if (!__percpu_rwsem_trylock(sem, reader))
129                 return 1;
130
131         p = get_task_struct(wq_entry->private);
132         list_del_init(&wq_entry->entry);
133         smp_store_release(&wq_entry->private, NULL);
134
135         wake_up_process(p);
136         put_task_struct(p);
137
138         return !reader; /* wake (readers until) 1 writer */
139 }
140
141 static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
142 {
143         DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
144         bool wait;
145
146         spin_lock_irq(&sem->waiters.lock);
147         /*
148          * Serialize against the wakeup in percpu_up_write(), if we fail
149          * the trylock, the wakeup must see us on the list.
150          */
151         wait = !__percpu_rwsem_trylock(sem, reader);
152         if (wait) {
153                 wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
154                 __add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
155         }
156         spin_unlock_irq(&sem->waiters.lock);
157
158         while (wait) {
159                 set_current_state(TASK_UNINTERRUPTIBLE);
160                 if (!smp_load_acquire(&wq_entry.private))
161                         break;
162                 schedule();
163         }
164         __set_current_state(TASK_RUNNING);
165 }
166
167 bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
168 {
169         if (__percpu_down_read_trylock(sem))
170                 return true;
171
172         if (try)
173                 return false;
174
175         trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
176         preempt_enable();
177         percpu_rwsem_wait(sem, /* .reader = */ true);
178         preempt_disable();
179         trace_contention_end(sem, 0);
180
181         return true;
182 }
183 EXPORT_SYMBOL_GPL(__percpu_down_read);
184
185 #define per_cpu_sum(var)                                                \
186 ({                                                                      \
187         typeof(var) __sum = 0;                                          \
188         int cpu;                                                        \
189         compiletime_assert_atomic_type(__sum);                          \
190         for_each_possible_cpu(cpu)                                      \
191                 __sum += per_cpu(var, cpu);                             \
192         __sum;                                                          \
193 })
194
195 bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
196 {
197         return per_cpu_sum(*sem->read_count) != 0 && !atomic_read(&sem->block);
198 }
199 EXPORT_SYMBOL_GPL(percpu_is_read_locked);
200
201 /*
202  * Return true if the modular sum of the sem->read_count per-CPU variable is
203  * zero.  If this sum is zero, then it is stable due to the fact that if any
204  * newly arriving readers increment a given counter, they will immediately
205  * decrement that same counter.
206  *
207  * Assumes sem->block is set.
208  */
209 static bool readers_active_check(struct percpu_rw_semaphore *sem)
210 {
211         if (per_cpu_sum(*sem->read_count) != 0)
212                 return false;
213
214         /*
215          * If we observed the decrement; ensure we see the entire critical
216          * section.
217          */
218
219         smp_mb(); /* C matches B */
220
221         return true;
222 }
223
224 void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
225 {
226         might_sleep();
227         rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
228         trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
229
230         /* Notify readers to take the slow path. */
231         rcu_sync_enter(&sem->rss);
232
233         /*
234          * Try set sem->block; this provides writer-writer exclusion.
235          * Having sem->block set makes new readers block.
236          */
237         if (!__percpu_down_write_trylock(sem))
238                 percpu_rwsem_wait(sem, /* .reader = */ false);
239
240         /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
241
242         /*
243          * If they don't see our store of sem->block, then we are guaranteed to
244          * see their sem->read_count increment, and therefore will wait for
245          * them.
246          */
247
248         /* Wait for all active readers to complete. */
249         rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
250         trace_contention_end(sem, 0);
251 }
252 EXPORT_SYMBOL_GPL(percpu_down_write);
253
254 void percpu_up_write(struct percpu_rw_semaphore *sem)
255 {
256         rwsem_release(&sem->dep_map, _RET_IP_);
257
258         /*
259          * Signal the writer is done, no fast path yet.
260          *
261          * One reason that we cannot just immediately flip to readers_fast is
262          * that new readers might fail to see the results of this writer's
263          * critical section.
264          *
265          * Therefore we force it through the slow path which guarantees an
266          * acquire and thereby guarantees the critical section's consistency.
267          */
268         atomic_set_release(&sem->block, 0);
269
270         /*
271          * Prod any pending reader/writer to make progress.
272          */
273         __wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
274
275         /*
276          * Once this completes (at least one RCU-sched grace period hence) the
277          * reader fast path will be available again. Safe to use outside the
278          * exclusive write lock because its counting.
279          */
280         rcu_sync_exit(&sem->rss);
281 }
282 EXPORT_SYMBOL_GPL(percpu_up_write);