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Linux/kernel/locking/percpu-rwsem.c

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  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);
271 

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