| // SPDX-License-Identifier: GPL-2.0-only |
| #include <linux/atomic.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/sched/task.h> |
| #include <linux/sched/debug.h> |
| #include <linux/errno.h> |
| #include <trace/events/lock.h> |
| |
| int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, |
| const char *name, struct lock_class_key *key) |
| { |
| sem->read_count = alloc_percpu(int); |
| if (unlikely(!sem->read_count)) |
| return -ENOMEM; |
| |
| rcu_sync_init(&sem->rss); |
| rcuwait_init(&sem->writer); |
| init_waitqueue_head(&sem->waiters); |
| atomic_set(&sem->block, 0); |
| #ifdef CONFIG_DEBUG_LOCK_ALLOC |
| debug_check_no_locks_freed((void *)sem, sizeof(*sem)); |
| lockdep_init_map(&sem->dep_map, name, key, 0); |
| #endif |
| 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); |
| |
| static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem) |
| { |
| this_cpu_inc(*sem->read_count); |
| |
| /* |
| * 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 sem->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 sem->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 !sem->block the critical section starts here, matched by the |
| * release in percpu_up_write(). |
| */ |
| if (likely(!atomic_read_acquire(&sem->block))) |
| return true; |
| |
| this_cpu_dec(*sem->read_count); |
| |
| /* Prod writer to re-evaluate readers_active_check() */ |
| rcuwait_wake_up(&sem->writer); |
| |
| return false; |
| } |
| |
| static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem) |
| { |
| if (atomic_read(&sem->block)) |
| return false; |
| |
| return atomic_xchg(&sem->block, 1) == 0; |
| } |
| |
| static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader) |
| { |
| if (reader) { |
| bool ret; |
| |
| preempt_disable(); |
| ret = __percpu_down_read_trylock(sem); |
| preempt_enable(); |
| |
| return ret; |
| } |
| return __percpu_down_write_trylock(sem); |
| } |
| |
| /* |
| * The return value of wait_queue_entry::func means: |
| * |
| * <0 - error, wakeup is terminated and the error is returned |
| * 0 - no wakeup, a next waiter is tried |
| * >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive. |
| * |
| * We use EXCLUSIVE for both readers and writers to preserve FIFO order, |
| * and play games with the return value to allow waking multiple readers. |
| * |
| * Specifically, we wake readers until we've woken a single writer, or until a |
| * trylock fails. |
| */ |
| static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, |
| unsigned int mode, int wake_flags, |
| void *key) |
| { |
| bool reader = wq_entry->flags & WQ_FLAG_CUSTOM; |
| struct percpu_rw_semaphore *sem = key; |
| struct task_struct *p; |
| |
| /* concurrent against percpu_down_write(), can get stolen */ |
| if (!__percpu_rwsem_trylock(sem, reader)) |
| return 1; |
| |
| p = get_task_struct(wq_entry->private); |
| list_del_init(&wq_entry->entry); |
| smp_store_release(&wq_entry->private, NULL); |
| |
| wake_up_process(p); |
| put_task_struct(p); |
| |
| return !reader; /* wake (readers until) 1 writer */ |
| } |
| |
| static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader) |
| { |
| DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function); |
| bool wait; |
| |
| spin_lock_irq(&sem->waiters.lock); |
| /* |
| * Serialize against the wakeup in percpu_up_write(), if we fail |
| * the trylock, the wakeup must see us on the list. |
| */ |
| wait = !__percpu_rwsem_trylock(sem, reader); |
| if (wait) { |
| wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM; |
| __add_wait_queue_entry_tail(&sem->waiters, &wq_entry); |
| } |
| spin_unlock_irq(&sem->waiters.lock); |
| |
| while (wait) { |
| set_current_state(TASK_UNINTERRUPTIBLE); |
| if (!smp_load_acquire(&wq_entry.private)) |
| break; |
| schedule(); |
| } |
| __set_current_state(TASK_RUNNING); |
| } |
| |
| bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try) |
| { |
| if (__percpu_down_read_trylock(sem)) |
| return true; |
| |
| if (try) |
| return false; |
| |
| trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ); |
| preempt_enable(); |
| percpu_rwsem_wait(sem, /* .reader = */ true); |
| preempt_disable(); |
| trace_contention_end(sem, 0); |
| |
| return true; |
| } |
| EXPORT_SYMBOL_GPL(__percpu_down_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. |
| * |
| * Assumes sem->block is set. |
| */ |
| 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 __sched percpu_down_write(struct percpu_rw_semaphore *sem) |
| { |
| might_sleep(); |
| rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); |
| trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE); |
| |
| /* Notify readers to take the slow path. */ |
| rcu_sync_enter(&sem->rss); |
| |
| /* |
| * Try set sem->block; this provides writer-writer exclusion. |
| * Having sem->block set makes new readers block. |
| */ |
| if (!__percpu_down_write_trylock(sem)) |
| percpu_rwsem_wait(sem, /* .reader = */ false); |
| |
| /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */ |
| |
| /* |
| * If they don't see our store of sem->block, then we are guaranteed to |
| * see their sem->read_count increment, and therefore will wait for |
| * them. |
| */ |
| |
| /* Wait for all active readers to complete. */ |
| rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE); |
| trace_contention_end(sem, 0); |
| } |
| EXPORT_SYMBOL_GPL(percpu_down_write); |
| |
| void percpu_up_write(struct percpu_rw_semaphore *sem) |
| { |
| rwsem_release(&sem->dep_map, _RET_IP_); |
| |
| /* |
| * 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. |
| */ |
| atomic_set_release(&sem->block, 0); |
| |
| /* |
| * Prod any pending reader/writer to make progress. |
| */ |
| __wake_up(&sem->waiters, TASK_NORMAL, 1, 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); |